That should be trivial. I've never laid hands
on an RC25, but there
must be a microswitch that detects when a cartridge is installed, and
is accessible for bodging...?
Yes, and don't do it!
In every fixed/removeable drive I have worked on (OK, never an RC25, but...)
the heads for both the fixed and removeable disks are on the same positioner
carriage. With the result that if you load the heads without a removeable
cartridge in place, those heads will touch each other, which is a good
way to ruin them.
-tony