This type of bias arrangement (capacitor feed from HV sec.) was
common in famous HiFi amps, like Grommes in the '50's. The coupling
capacitor's value sort of sets the max available voltage.

Since the voltage is already in the area you need it for the Pignose, I
would try substituting the final 56k shunt to ground with a 25kB pot
above a 47k fixed resistor. This will allow you to vary from about 82k
(larger neg. output voltage) down to 47k for lower voltage, which may
allow you to run the tubes with a safe increased current. These values
of resistors would be my first try. You'll have to carefully experiment
before mounting the proper values on your fresh pcb.

As for a HV secondary fuse, just break the run on the high side of the
winding, going to the bridge rectifier (but after the bias takeoff if
possible), and insert your 250ma fuse holder.