Solutions of these three items should last indefinitly, and I bet once you worked out the process, it would take less time to mix them than to dissolve the solid. Also, the solid ferric oxalate has a finite shelf life and a dubious purity, I am guessing that the fresh mixed stuff would be significantly purer than the redissolved solid. It's not cheap either.
Making FO is a little bit more involved judging from the literature I have seen, for example the B&S method uses ferric ammonium sulfate to which you add ammonium hydroxide, to the resulting product you add oxalic acid to make ferric oxalate:
2Fe(OH)3+3H2C2O4 (H2O)2---------->Fe2(C2O4)3(H2O)6 +6H2O
Simple, but as you see the components are not that easy to get, specially here in Mx.
I have seen other methods where they add Ferrous ammonium sulfate to OA and then Hydrogen peroxide to oxidize the iron to ferric, but again, you have heat generation and efervecense, not really something I want to do in the darkroom.
The most dangerous I have seen was adding Ferric Nitrate to OA to make Ferric Oxalate and nitric acid, not something I want to do at home!
Your idea might have merit, using Ferric chloride and OA, wich would yield OA and Hydrochloric acid and the sodium bicarbonate as a buffer, I suppose if I check enthalpy tables I could see if the reaction proceeds, but as you can see this is becoming more and more involved and for me it is easier to just buy it new. Specially with the customer service B&S is providing, when I called them not only was Kevin Sullivan very helpful but offered to send me a new bottle free of charge!
FO is actually very stable, in the sense that it lasts years if kept closed tight and at normal humidities.
I know Terry King usually teaches in his workshop how to make FO, but I think this is one of those cases where conveninence outweights making it.
Thanks for your input and of course if one day you do decide to make pt/pd prints and make your own FO let me know how it went. http://apug.org/forum/html/emoticons/tongue.gif