You are right the switcher would be fairly easy (just bring up and down stratum instances on the same port but connected to different coin daemons and databases). If you wanted to be really slick you could just build it into stratum and keep the same sockets open, miner diff's, etc) and I also agree you would end up with a good bit of rework in the stats and balance/payout stuff (ie not as simple as just switching out /public/include/config/global.inc.php for each "current mining" coin) multipool just makes you configure payout addresses for each coin and shows stats for each coin separately.
think the simplest and most user friendly thing would be to write a cron orchestration piece that would based on some criteria wake up and automatically switch the current running stratum / or the coin it is working for to whatever is called for at the time.
Then use mpos as a base for the end user facing piece and set it up with bitcoin for payout and then make your own autosell/payout system and post stats like multipool and have another component that shows the sell (ie exchange, rate, exchange fee, etc).
Not trivial but not terrible I don't think.
not tried to do it yet and if I do I will post up my github
Seems most folks don't even want to talk about setting pools up much less modifying and extending code for what's out there! I would like to see that change!
Slush's stratum and mpos are awesome specimens, wish there were more options.