I agree with Chris (COH), the lack of response is because nobody is working with this one much, and those that have tried have not worked out the problems with it - yet. Chris is trying, and I have dabbled with it too.
You are in New Mexico, I think your purple flowered tree is Robinia x margarettiae (hispida x psuedoacacia) if it is growing as a human planted landscape tree. The hybrid x margarettiae can propagate itself by seed and by root sprouts, so if any of the hybrid were planted in the area your purple locust is likely this hybrid. The hybrid's flowers can range from nearly white to very dark purple pink. The darker color forms are the ones most often planted on purpose.
If it is growing in a "wild place" where an escaped introduced species is unlikely, then it is probably Robinia neomexicana - the New Mexio Locust. Its flowers tend to be a soft pink to almost white.
Regardless, they all grow with a similar pattern as Robinia psuedoacacia.
I've been "playing" with one or two off and on over the years. I have root suckers from a neighbor's tree sprouting everywhere in my back yard. I have left a few get some size, then dig them up. Without fail, all but one of my attempts died over the first winter after digging them up. This year the one that made it I brought into the same area I winter my Satsuki azalea, which is always above freezing. The top of the tree died back, but one branch sprouted from the trunk. It is looking rather pathetic at the moment. I think getting a well enough developed root system on a dug up tree, so that it can survive our cold winters is a problem. (zone 5b) I also think that if pruned hard, rather than the branch back budding, the tree may just abort the branch and grow somewhere else. If too much is trimmed from a trunk, it will send up a bunch of suckers from its roots instead of putting the energy back into the same trunk.
None of these issues say it can not be used for bonsai, but it does require planning ahead. You are in New Mexico, so your climate may be more to Robinia's liking. COH and I are near the northern limit of where this species does well. Its original native range was strictly a southern tree, but it has been spread all over the US, as both an ornamental and for wind break, firewood and waste land reclamation projects. It has been used to colonize defunct strip mines. A really tenacious tree in the 'wild'. I don't understand why it does not seem to like life in a bonsai pot.
I once saw an impressive trunk (maybe 5 inches diameter or more, 2 or 3 feet tall) that was collected, shown at a local bonsai show sometime back in the late 1980s, before digital photography. It had nice deeply fissured black bark. But I never saw the tree again in subsequent years. Usually at the Chicago MBS show at the Chicago Botanic Garden good trees do make return appearances every few years. Have never seen another Robinia at that show again. Robinia is a weed tree everywhere in my region. It is odd you don't see it as bonsai.
I love the fragrance of the flowers. It would be nice if someone could figure out what "the trick" is.
