It's an interesting idea and it could be possible although how it will perform is another question.

I too have been attempting an espalier, but with pyracantha, and in my zone 5 yard, the pyracantha dies back to snow level each year, and wisteria is borderline here as well. But, I think we should be able to use anything that blooms on old wood that is hardy for our area and spur prune it to shape. I would actually use some ornamental crabs, but the deer would like the buffet a little too much. I'm thinking of some upright cotoneasters or ribes speciosum, maybe lonicera nitida or maybe whatever I can find that will fit the critera of hardy, old wood bloomer.
If you decide on the lilac let me know how it goes, it will likely sucker if you are cutting it back very much.
Any of you more experienced growers out there, please let us know what you use. Thanks Shel