For pickling onions I use an old Good Housekeeping method of wet brining them for 24 hours/overnight in the ratio1 pint of water, per pound of onions per 2oz salt. Drain the onions, rinse and pack into jars. Cover with whatever vinegar you like, but do not dilute it with water. I usually use malt vinegar, but you can use a proportion of balsamic or sherry. I also use spiced vinegar - malt vinegar I have boiled with pickling spice, and left to go cold and infuse. When using this I put mainly malt vinegar in the jars and top up with spiced. To get really spicy pickled onions put a couple of dried chillies in the jars.
To answer your question Zoglet, I don't know, but if you don't try you're not going to know either.
I'm a big believer in 'nothing ventured, nothing gained'. ;D
Give it a go.
valmarg