The proposal by Fabrice looks better to me
-net usernet,macaddr=00:11:a:0:2:19 -net tunfd=10,macaddr=00:11:a:0:1:19
this way the syntax can easily be extended with new options as needed,
and the exact order of things is not important (you just specify the net
parameters as needed).
Only objection is that for the tunfd case I would use
-net tap,fd=10,macaddr=...
For the tap type we could have a 3rd optional parameter for the name,
e.g. -net tap,macaddr,name (with name defaulting to tapX if its not
specified).
What about this:
-net tap,ifname=qemu0,macaddr=...