IIRC, there's one shown in the NRA Gunsmithing Guide, Updated.  Essentially it involves putting a block on the bottom rear of the triggerguard for a push-through type safety, like is used on Remington shotguns, to block the trigger.  Same system Sportco used in Australia.

David
Montezuma, IA

I also don't see a need for a safety on a martini and I hunt with them.  But I also understand why some want them.  

several years ago I picked up this clunker Cadet for a project gun that has a sliding safety on it.  Here's what it looks like.

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I took the safety apart this morning for these pictures so it's the first time I have seen the insides.  It looks like it is missing a spring and ball, but you get the idea,

I found pictures of another safety on my old computer. 

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Something else you can do is to get your gunsmith if he knows how to do it, is to alter where the block comes to rest in the lowered position.
My latest BSA Martini is a model 10 and the loading lever is taken all the way down to load a round, in this case a 7mm Waters and then the pressure on the lever is released slightly and the block in the down position will come to rest so that the case will not fall out of the action. You can also put the case not all the way home and it still will not fall out if you want to carry the rifle like this.
I have only just picked this rifle up at the weekend from my gunsmith and have yet to fire it.
The stock is a more or less a copy of how Holland and Holland used to stock their rook rifles.
Twenty two inch octogon barrell and it weighs in at five and a half pounds. 
I am getting a bit old and wrinkly now so it should be a bit less tiring to carry when bush bashing for ten hours and never mind the re-coil.
Not sure if I know how to load more than one photo so might have to do another post.
Regards.

Nice looking rifle and done by a Gunsmith who know how to adjust the Martini Action properly.   

Lucky you.

Its the gunsmith that built the rifle who has all the good taste and knows what a rifle should look like and his name is John Hall who is a Kiwi but who lived and worked in South Africa as a gunsmith for about twenty or thirty years or maybe even more as he was in the S/A practical pistol team, for  the event that was held in Rhodesia in the seventies for the first world practical pistol shoot. I had a friend in England who went to this shoot and he did a video which he showed on a club night.  
I suppose you could say I have grown up now as these days I only shoot single shot rifles. I believe John also came out top one year in a world black powder rifle shoot in the USA. 
He's been back working in NZ for about seven years now

Nero;

That is a beautiful martini.  I copied the picture, and have it as wallpaper on my computer for inspiration.

I have a few martinis in cartridges based upon the 30WCF cartridge, and really like them all.

Do you suppose that your stockmaker might export a stock to the States?

Edbu
Washington State
USA (the one south of Canada)