[Arran]

That reminds me, someone gave me a box of some old copies of the Antique Radio Classified. In several of these old magazines there were ads for some sort of solid state substitute for 01As, 99s, etc., that fitted across the pins of the original tubes. I would guess that they used some sort of field effect transistor, and some peripheral components on the wafer on which the substitute was constructed. Of course if the tube is missing altogether these would not be all that useful.
 The 6C4 obviously works, and gets the job done, you may also be able to run the filaments on 6.3 volts AC and skip the battery eliminator,  I'm just not that crazy about the appearance, although at least it's still a vaccuum tube and not "sand". I may even have some in my TV tube stash already, that or 6AQ6s, a 6AV6/6AT6 would be another option, if you disregard the diodes. I read an article about battery tubes substitutes in the Old Timer's Bulletin, but this was with regard to replacing WD11s or WD12s, the 1G4 was one that was suggested, but they preferred to use a triode connected 1C5 or 1Q5, all are 1.4 volt battery types by the way, easy for a Radiola 3 or 3A, but more work for an A.K or a Freshman Masterpiece.
Regards
Arran