You certainly can, i wrang the DVLA and asked them, i had to deliver a car to someone in swindon and only needed insurance for a few hours (www.dayinsure.com), but obviously needed tax too so had to tax it with my 1 day cover note...they said it was fine.
Definately can, we offer 7 days free insurance to all customers from our branch which is emailed directly to ourselves and we use the 7 day note to tax all our vehicles.