The amp I just built uses 12AX7s and EL34s, but my stock of both is almost dry. If I were to build another, I'd use a couple 5755 up front (low microphonics) and 6N2P-EV for the remainder of the gain chain and PI. At the output, I guess $40 or $50 for a pair of EL34 isn't too bad, but it's still possible to buy four 6P3S-E for that price. I have a set of those running in a 40W/ch stereo with almost +500V on the anodes. They're excellent tubes.
All these tubes are plentiful and (for now) inexpensive. They typically last thousands of hours, and carrying a couple spares on the road isn't exactly a hardship. So why isn't there more interest? Is it just a matter of continuing to use what we know? Even the commercial manufacturers seem afraid to offend customers by moving away from traditional types. There's a whole world of really good tubes out there. Considering all the interest in DIY, and the fact we can build whatever we want, it doesn't make sense for everyone to be stuck in such a rut.
Jack
The 6AU6 wouldn't be my choice for this. Anode impedance of the EL34 is about 15K - 20K ohms. The 6AU6, like most small-signal pentodes, is closer to 1 megohm. That's problematic, because loading affects tone (harmonic content) and breakup characteristics. It's impossible to load the 6AU6 in the same way as a EL34 or EL84 power pentode. Whether the 6AU6 might sound like an EL34 is anyone's guess, but as you noted, the 7-pin socket limits your options if it doesn't work out. On this point, the 12AU7 can't be easily subbed either, because it's two triodes in a single envelope. Push-pull pentodes will require two sockets. If you think you'll build with the 12AU7 but maybe try pentodes later, you should include an additional socket hole in the chassis at the outset.