One neat thing about PROPERLY TUNED modern galvos, if not fed ultra sharp high frequency square waves, they tend to survive anything you throw at them, especially if there are size pots between the signal source and the galvo amp. A dual 10K or 100K audio taper pot makes a great XY size control. The one radio shack sells for 3$ will enable your experiment.
A few years ago before a recent price fall in some controllers, I looked long and hard at a Zektor(tm) vector controller, but would have needed some one to write some Windows code. I had a skilled person in mind who could write that code, but he turned out to be a real failure when working with others and I canceled the project. The Zektor folks were not so keen on trying,and I did not feel like loaning them a 700$ pair of galvos at the time, so the project stopped. But at the time I did not see any obvious problems using the Zektor for laser with a few minor mods.
What I'd saying is it can't hurt to try if you keep the initial angles down using the size controls.
Newer galvo amps, as my Friend Andrew often reminds me, are more PII, then PID, so they tend to not launch into oscillation with rough drive waveforms.
Start with a circle. Work up from there.
Worse comes to worse you may need to play with/remove/increase your integration cap on your generator.
As to just turning the speed down, yes, just expect to perhaps have to do some amplifier tuning on the small jumps. Its not a big deal, three 10 turn trim pots. Since you already have a oscilloscope, one of the techniques we use is to tune one axis on a square wave for the least ringing and overshoot, while watching the feedback signal. So you can match velocities if you need to, but I bet you can just lower your speed and make changes to the image. So what, if you loose 10% of the scanner's performance, no big deal. In reality everybody opens up the angle and reduces the scan speed, ILDA 30K is really a data interchange format if you think about it. You tune to 30K/8' to share images correctly, more then anything else. When you run a show you most likely open up to 15-20' and the PPS rate comes down, mathematically the angle multiplied by PPS is roughly a constant, so as you increase angle, point rate falls off. Its not directly a linear function, but it does happen.
For those wondering what a Zektor is:
http://www.zektor.com/zvg/downloads.htm
Steve