Josh, it is totally fun and cool, but I'd rather err on the side of more vigor and get some extra mulch out of the deal.
This loquat has been in ground for 3yrs, and was about 4-5' tall and wide when planted. Now I'd say it is about 8' tall at the tallest part and roughly 12-15' wide. I cut out any vertical growth in the middle of the tree once or twice per year. I have yet to do much pruning to control width, but now that the fruit is mostly off of it for this round I'm going to cut it back a good deal, and open up one third of it around where I grafted a BigJim scion.
To answer the original question, I'd just use loquat for loquat rootstock. It is cheap and readily available this time of year and handles drought well. As long as you aren't shy about pruning it, it can be totally manageable size-wise and the leaves make a good mulch.
My loquat stays plenty dwarf with pruning once or twice per year, no fancy rootstock required.
How old is your tree? I think it can be managed if trained small from the start and pruning schedule maintained, but if left unchecked, it will grow to a medium sized tree, like 18 to 25 ft and wide...hard to harvest like that.