I dont know what the new toy is but the short answer is no. For almost any usage an A-C generator needs to run at the proper RPM so the hertz is correct.
Long answer, With A-C current two things are important for most loads, frequency and voltage. When you slowed the shaft speed own you changed the frequency or hertz which screws up any motor load you put on it and possibly electronics. Light bulbs and resistance heaters dont care, The AVR is doing what it is supposed to do, keep the voltage at up, will it damage it over time? Possibly.
If your desires are for a slower running engine you to change the drive ratio if belt driven, this allows you to slow the engine down but keep the generator at proper RPM If direct coupled your stuck at 3600 RPM for 60 hrtz. You set it just above 60 with no load and under fullload it will drop to 58-59 with most set ups.