You could always use a current sensing relay on the compressor power feed, and use it to operate your hourmeter at whatever voltage you like. I have used this one with good results:
http://www.crmagnetics.com/newprod/ProductView.asp?ProdName=CR4395 I think Red Lion Controls makes an LCD hourmeter that runs off an internal battery (5-10 year life, IIRC), and requires only a contact closure for input. One of these with a current sensing relay might be just the ticket...
