Even if the leaves emerge a month or month and a half before bloom they will need sun during and after bloom to produce food for the bulb.

How about potting them? During bloom it can be placed for enjoyment and the remaining 'leaf time' be placed in full sun. I have success doing that with the smaller ones that accept our climate.

:: Even if the leaves emerge a month or month and a half before bloom they will need sun during and after bloom to produce food for the bulb. ::

Actually, no. That's how it works for iris, but not for most of the 'big ball' ornamental alliums. The allium foliage gets all its sun and feeds it back to the bulb before bloom; the blooms and their stalks start to senesce and dry out immediately after bloom. So they're fine for a site that's sunny from late March until early May, as long as the soil is well drained. Rot is the enemy, not necessarily shade.