I keep white clover as a living mulch for brassicas, burdock, jerusalem artichoke, and other things that are mostly above 6"... I don't think it's so important to worry if nitrogen is released in summer or winter - just treat it as a long term gain..
Asparagus seems like a great fit for living clover mulch! I had a question on raspberries if you've seen it: are new sprouts able to come through pretty easily? I've been using
wood chips to mulch raspberries, but maybe I
should consider living clover mulch?