I think I installed the Digispark code onto an ATtiny85. I did not disable the reset pin. It wasn't required just to make it work.
There are other signals too, like /WR to write something (eg. a fuse), /OE to read from the chip, and PAGEL to lock data into the page buffer.
HVP must be intended for "high speed" uploading and verification for new processors that have not yet been installed.
So are you saying if I screw up the fuses...
Is it just that most boards don't give access to the XTAL pins...
...or would the signal not work if a crystal or resonator were installed.
I assume a crystal/resonator and the capacitors would interfere. I have no idea if they would prevent HVP from working.