RE: anycasting behind different ASNs?

Are there any practical issues with announcing the same route behind different ASNs?

No.

Shortly I'll have two seperate sites (EU, US) announcing their own space behind their own ASNs but have a desire to anycast a particular network out of both locations as well.

(This is just my attempted to now have to deal with GRE tunnels between sites that aren't logically connected anyways and using the same ASN).

Check 192.88.99.0/24. It is an anycasted prefix for 6to4 tunneling. No AS number was assigned for 6to4, thus it has inconsistent AS origin, and works without any problems. Regards, james