Hi,

It's unfathomable that any hospital would implement a policy that was known to put at risk the wellbeing of a mother and baby.....just "because it is policy".
You are doing a great job being so proactive in this. Whist expressing will stimulate the release of oxytocin, this will not act on the uterus unless the uterus is "ready" to go in to labour - so don't worry about this. Often mothers are warned against expressing, when no-one would consider asking them to refrain from sex and orgasm, which also releases oxytocin.
The quantities of colostrum are going to be small, though it varies from woman to woman. Be gentle with yourself - nipples can be sensitive at this time.

The Australian Breastfeeding Association have a good paper on antenatal expressing - give them a call. It's by Sue Cox and is from their journal "Breastfeeding Review"