To be honest, I am not really for sure wraggster. Unless the code was changed, I believe it uses the variables such as the tagcloud to call upon the database it's attached to... so unless the database was shared among the forums I don't know if it's possible. Then again, this is a bit beyond my expertise, if you try asking over at network.zoints.com you might get a bit more confident answer from one of our engineers... but I do believe that it is not something easily done right out the box.
Glad to hear you love the tags though.
- Reid