I've not tried this sort of thing in mpagd, so can only give general advice based on my old speccy knowledge.
If you want to print a UDG out the easiest ways is chr$(value) in BASIC. The value being the character set value. Normally 128 + as the speccy's graphics options started from there. But it should be a matter of finding the char value of the udg you are using and printing that. Not sure how you chase around mpagd for that, but is basically what you need.