Lilacs season is very short, between two to four weeks each year. If the winter is warm they don't even flower at all.
Even using different varieties to spread the season to six weeks you would still have 11 months to enjoy the smell of manure. if is the unique candidate, is a poor choice.
So, nope. Definitely not useful. We have much better options
In the winter, it's cold outside, and everything is frozen. So you don't need to hide the odor. Same with the fall. The lilac is easy to grow, and nice in the spring. So that just leaves the summer, but not many bushes are producing scent in the late summer, anyway.
If an outhouse was nearby in the past, there was a pretty big hole associated with it which may still be around. Might not be pleasant to discover by accident...
