Kia ora Tony,
I do not see this as a problem and in fact even as a requirement. Having applied a set load restraining the engine to say 4,000 RPM, would you not judge alterations made in respect of tuning as being a indicated by any increase/decrease in RPM against the fixed load.? If 4,000 RPM was the exact engine speed aimed at, the load could in stages be manually adjusted as required.
As I see it holding the RPM by automatically applying throttle, would defeat any basic, direct and positively recorded result during tuning. In any event, surely it would be best to consider such a complication, only after everything basic is operating correctly with no frills.
You are chewing well after biting off a hefty challenge.
Congratulations, Trevor.