Hi Tara,
This would be a little tricky to implement. I suppose we would need to use a ListMail customization along with some trickery to define which site should have which "from" name and address.
An easier workaround could be with multiple lists but finding a way to simplify certain aspects of it. For example, a simple custom script could be used to create 100 lists with the same settings apart from the "from" name and email. You might also get away with storing message (welcome, followups, etc) subject and text in "custom text" message codes, using those codes for each list's messages. Of course, if you have followups, this will lead to a LOT of redundancy and may not be optimal, especially if you plan to add more followups. Like the Custom HTML feature that was recently modularized, I should also modularize followups so that this is no longer an issue and it is easy to apply a single sequence to multiple lists...
Do you have followups? This seems to be the biggest stumbling block to the 'easy' approach. Let me know if it might work for you. If it seems like too much hassle, I don't mind investigating the more difficult approach, too.
As I mentioned, it would take a little trickery.. a custom modification to ListMail (hidden feature:-) possibly a config file storing the names and email addresses to send from for each site.
Regards