i would consider the lilac much hardier than a clem ... so it sounds like a good idea to me .... i would only use the leave to the extent to bury the pot;.... so winter sun.. doesnt heat the pot in mid winter ... the plant itself.. should be not buried all that much ... the key is.. get it dormant and KEEP IT dormant ... and with pots.. its a matter of insuring the roots dont warm.. mid winter.. and the plant comes out of dormancy .. and then it gets cold again .... holding over pots .... is holding over pots ... it shouldnt really matter what zone appropriate plant is in the pot .... the rules should not vary all that much .. short of zone pushing and truly exotic plants .. and lilac is not exotic ... lol is a sucker still a sucker.. once you have removed it from the mother plant??? dont limit your search for info ... based on it being a sucker ... imo.. its now a lilac ... if you see the difference ... dont focus on the wrong descriptive word ... ken

the term is rejuvenation pruning ... here is a link of vids ...: https://duckduckgo.com/?q=rejuvenation+pruning+lilac&t=ffcm&iax=videos&ia=videos interior wood does not heal like a broken arm might... as such.. staking it up is useless ... if that is your neighbors house.. why are you messing with it??? ... just helping ... ???? it is a full sun plant.. the fact that it is leaning away from the house.. means its stretching for the sun ... and that is the base cause of what the cause was ... i also suspect ... that trees planted decades ago.. when this monster was planted.. have now decreased light .. is it in shade part of the day.. other than from the house ... prune after blooming.. use the flowers on the ground for indoor cut flowers .. but wait ... if you want straight flowers.. if you can rig it upright somewhat ... it might help flower development.. for the short term ... otherwise.. after bloom ... start the rejuvenation process.. which is usually a 2 or 3 year process ... but you can also do it all at once ... imo ... with no insult to the root mass.. you might be amazed how fast it regrows this season ... ken

Google 'rejuvenation pruning'. It is an excellent way to "bring back" overgrown or lanky caning shrubs, which is what a lilac is. You selectively remove a third of the oldest canes or stems as close to the ground as possible. This will encourage new and vigorous growth from the root crown. If you do this in each of the three years - removing a third each year - then you will have pretty much a brand new lilac and with a shorter, more manageable height.

Even though a very cold hardy shrub, because these hybrid lilacs bloom earlier than the common lilac, it may be the flower buds are being damaged by late freezes. Other than improper pruning or excessive fertilization (no need for the uric acid and could be detrimental to flower development). there aren't many reasons for a lilac not to bloom. They are not biennial in that regard. Biennial fruit production is much different. Some varieties are biennial by nature or genetic make-up and others can be tiggered into biennial production by stress. Flower and bud damage by late frosts/freezes also contribute heavily to lack of fruit production in some years and this could very well be a similar issue wth your lilac.