if reloading a cartridge that has no "published" data, would using published data for a cartridge that the same case capacity (gr of water) be safe? 10.4 x 38R is the target of this question.

I.E.  38-55- (cc 52.72) 26.5gr IMR3031 - 16,400 psi, 335gr bullet
      10.3 x 38 R - (cc 53.45) 26 IMR3031 <20,000, 330gr bullet

When nothing else is available I say that is a good first approach but best of the other cartridge has a similar bullet diameter. In addition to the initial chamber volume the peak pressure is also going to be dependent upon the rate the volume increases as the bullet starts down the bore. With a larger caliber/bullet diameter the volume of the combustion space expands faster than with a smaller diameter for the same rate of acceleration. The acceleration too is dependent on the bullet weight, friction and engraving forces on the which drive different pressure rise hence you're not going to see a perfect match based on just case capacity but it's still probably the best thing to match (along with bullet weight) when comparing two different cartridges. 

I do have some load data from Donnelley's Cartridge Conversion book on the 10.4 Swiss - it's not 3031 but should give you an upper bound as 4198 is faster powder.