Howdy.
I too needed smtp for outgoing mail, since I absolutely refuse to configure sendmail to allow access for the user "nobody", which means if it's a domain that isn't on the same box, it's dropped.
Normally, I use the Coffee2Code SMTP plugin, and it's wonderful.
MU is it's own beast in some aspects, and allowing individual users all the options associated with the c2c plugin is a bit much. Not to mention that it needs to be set to use smtp.
So, I made a few modifications to the c2c plugin, and it's working like a champ.
It isn't anything fancy, and the default value settings (read as: your smtp info) simply replaces the default settings in the code.
I did spruce it up a bit, so that it auto-inserts their username as the "from" address, as well as the sender's displayed name too.
Oh, I also added in the nonce for the settings form, and took out all the settings options except for the from email address and displayed name.
I'll admit that I haven't checked in a couple months to see if they've updated their plugin, so the version I have may not be the latest.
Anyway, what I'm getting at is after I go back and check their license, if someone is interested in this plugin that's been edited to work (better) with MU, I'd be happy to share it.
If their license doesn't allow redistribution, I'll work with them and pass along the changes I've made so they (hopefully) can opt to release an MU version.
Worst case, I can at least mention what I changed and hopefully not make anyone upset.
For the record, the original plugin works correctly, unmodified and in the mu-plugins directory (all 4 files), but individual blog owners get the full options page.
As a note, it could also be arranged to remove the settings page all together, as it calls an action for the admin head that inserts the default data.