It can be done. We have two pairs of output tubes, EL34 in PPP. "Half power" switch can be incorporated in a way that it "switches out" one pair of output tubes, say tube 1 & 4. Best between cathodes and the ground - both pins 8 from tubes 1 & 4 soldered together and solder the switch (close to the tubes) between cathodes and the ground.
Triode/pentode switch - in "normal" or pentode mode pins 4 of all power tubes are connected together on "g2 supply", or through LC filter from B+. You must now connect it with a another switch in a way that in pentode mode all g2 pins (through it`s 1k resistors) are connected on this LC filter/supply, and in another switch position - every g2 (pin 4) connected to the it`s anode (pin 3)- on the same tube. In other words - on tube 1, pin 4 to pin 3 switch and in another switch position - pin 4 to the g2 supply. The same with other three tube.
"15W class A - one tube" - the best to disconnect two tubes, say 1 & 4, then disconnect in some way input AC signal on one tube (say tube 2), leaving negative "bias" voltage" in place, tube is still "idling". Then we`d "driving" with AC signal just the tube 3. It can be operated like triode, or pentode (see above). We have "quasy SE" circuit, with just one tube actually amplified, and other "balanced" DC "bias" current through the OPT.

Like I said - it can be done. Even it can be "footswitch" (relais) operated. But - are this easy and safe mods for "average" DIY-er?

P.S. You can find this and similar mods/schematics in "The Ultimate Tone" book(s) by K. O`Connor.