Can I presume that the sled/muzzle rest is attached
at the null point of the barrel? 
Or to put it another way, it seems to me the attachment 
of the muzzle rest may affect the harmonics of the 
barrel- favorably or unfavorably depending on where it is clamped on.
beltfed/arnie

That looks like a pretty typical of the simple solid effective DIY muzzle rests I have seen on the range. 
Some are more elaborately machined and/or of different materials. But all are usually more or less that size in terms of height, width, and length and shot off a similar front platform.
It's not really that different from the Pope version and operates on the same principles,  a bit larger and bulkier and with round stock for the "feet" rather than the flat base of the Pope device.
The Pope device we consider a muzzle-rest was the muzzle rest portion of a simple machine rest that incorporated an adjustable action support that allowed a positive repeatable return-to-battery.  It is mounted on a wood base that has a row of drilled holes extending back for the muzzle end that are tightly sized to hold a locator peg that is engaged by a notch on the front of the muzzle rest.  I believe that accurate brass/bronze replicas of the Pope front rest have been produced in the past.  I just missed out on picking one up at one of the EG matches last year.

I just found the earlier discussion that contains the  photos of the replica Pope rest. 
The photo of the muzzle-rest is with the 1st post on the top of the 2nd page.  
   
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num=1410028408/15

Note that allen-head caps crews were not used on it  

Re clamping position:   my experience to date has been with heavy stiff barrels on 22rf rifles.   On them, the position does not seem to be particularly critical. Ammo selection, conditions, and rifle holding technique have more impact on accuracy----and probably mask any variable created by the muzzle rest.   once I transition to using it for load devilment for centerfire offhand rifles I may find a different situation.

Edit:  I just noticed that ought-to correct substituted "load devilment" for my intended "load development"  but since it may well be more accurate than not I'll let the unintended humor stand

Assra can go no farther without adopting a limit on the width of a muzzle rest, I propose the width be no longer than the width of the bench (about 3 feet ) longer than this would probably wipe your buddy's bench off during recoil. How-about a 6 inch muzzle rest with outriggers.  Ledball

DW, you've got a handle on this muzzle rest thing, you're seeing bad things coming way down the road. What do you think about the outriggers, I thought it was a great idea.   Ledball

If sendo's original question is to compete with modern BR stocks with a traditional schuetzen stock, I would suggest making a wide forearm rather than using muzzle rests.  I can post a picture of my Ballard if that is the intention.  IMO, muzzle rests will always be at a disadvantage against modern style stocks. But, it is more important to shoot flintlock than to win matches  I'm sure that theory applies to schuetzen rifles, flyrods and dry flies too 

Sorry,   non-breechloaders are not allowed in ASSRA matches,    though I had been seriously tempted to borrow a Ferguson rifle from a buddy and show up with it at a match just to make heads spin .   Building a match grade target Ferguson is on my bucket list of projects.

Personal opinion/speculation:   I'm not sure that the outrigger sled-runner style rest won't create problems of their own when it comes to adjusting for conditions.  We tend to adjust for conditions by tweaking the front rest. 
With long flat runners with a long solid contact surface on a flat bearing surface I think it could bind on elevation adjustments.  
 As part of a semi-machine rest for ammo testing I think it would work well.  Under match conditions it might be different. 
  
My personal use, at the current time, is for ammo check and sight-in adjustment for offhand shooting only.  I simply want the stability they offer to my euro-schuetzens with their typical stock style.  Mine is small and light enough I can still pick it up and shoot offhand with it attached to the barrel.  It just helps make the rifle a little muzzle heavy--which I like.   a larger more massive muzzle rest would have to be removed before I could shoot it offhand.  that'd be a PITA and it might or might not affect the accuracy.  

to summarize my opinion. I believe that the muzzle rest adds most value to the offhand shooter who is using a schuetzen style rifle. 
Perhaps it may help someone who is shooting a traditional class rifle in the benchrest matches.  
I suspect it has little of value to add to the shoot who is shooting in a more "unlimited" (in the ASSRA/ISSSA etc context) bench-rest competition.
    again JMHO