beetree:
Nick there are non-backboard boards with pins, though at least some are the Jy-Mcu(?) things.
This is incoherent.
So assuming one uses a 3V3 supply , there's still the RX level which would be wrong?
Read the back of the board to assess whether using 3.3v is a good idea. There are HC-06s on 3.3v boards and HC-05s on dual voltage boards. And yes, you should still feed Rx with 3.3v.
Wouldn't be too hard to put two SMDs on a backboard, perhaps some do. Some of them appear to have different part counts.
If you want to go down that path, you are a better man than I. Mere mortals are more inclined to put the board and resistors on a shield, which is what the commercials do as well. You can get HC-05 with downwards pins thereby more suitable for this.
Looks like the Uno 3.3V is regulated by a LP2985, max 150mA
I'm afraid that means nothing to me, but this https://store.arduino.cc/usa/arduino-unos -rev3 does. Maybe there have been changes, but it's meaningless anyway. Nobody, but nobody, runs HC-0x off the Uno 3.3v pin, principally because it is a really dumb idea.