This is far too complex a subject to be addressed by Government decree like tax. Governments and other large organisations can only function because they are basically stupid and don't do emotion, like machines.
When it comes to a very emotive subject like this, great delicacy is needed and I tend to agree with Kvalidir when he says we have things about right as they are.
I have a donor card but I wouldn't expect others to carry one, any more than I would expect them to be creating life by procreating or expect them not to have abortions - these are very personal things that are not to be resolved by committee.