Ordinarily lilacs are pruned immediately after flowering. But in this case and flowering isn't an issue, could it be lopped off somewhere along the three branches in the hopes that an established root system will produce new shoots along the stumps with which to work? A smaller seedling would have been the ideal.

I would suspect you know infinitely more about the art of bonsai than I do - my experience with the technique is looking at them at the National Arboretum with a kind of spellbound wonder.

There is actually some show of back budding on the branches...out of the two I liked,this one shows potential without the chops showing that back budding.

A seedling would take a very long time to develop the base...the roots/nabari. Which is the first thing one looks at with bonsai. So chopping now won't cause die back this winter...that was my biggest concern. Phew! Thanks...

Just getting into bonsai...this would be a more untraditional one...Korean lilac more chosen for more dwarf like blooms and leaves. But, the base on this caught my eye.

Now the nursery girl mentioned that they see a plant not planted well when they see what I am desiring. Where in bonsai an old tree the roots push up and are noticeable...so Nabari/roots on a young tree gives it an appearance of an old tree.