Thanks for all the replies. Unfortunately, making my own solder would be a bit beyond my present resources. The good news is that the cracking on the butt plate tang was evidently very shallow and I believe I will be able to file it out.
I'm still wondering if adding solder to the edge of a brass inlay would be an effective way to correct small inletting gaps where you might otherwise glue in a tiny piece of wood. The thought being that you could add a bead along the edge and file it down. This idea comes from the process I use to solder on barrel lugs where a puddle of solder is formed on the barrel, filed down flat once cooled, then the lug is sweated on. Obviously you wouldn't want to do this to fill a gap so significant that the added solder distorts the shape of the inlay, but I'm talking about those tiny hairlines between metal and wood that can't be corrected by peening.
If the solder could be formed in this manner, I wonder how it will look once the brass begins to age. I get mixed results when attempting to glue wood in these situations with the stain being off, so perhaps trading one problem for another if the brass won't match perfectly.