I much prefer the speed of suretype over multitap, but when you go to type in anything that isn't in the dictionary it's a real pain. Unfortunately I have to do this often.
I heard in the old pearl OS there used to be an option for this by holding * but then they took it away in subsequent releases. Someone please tell me they came to their senses and added it back in somewhere into 4.5.
while you're typing your message-menu>"er" key should enable multitap.
I don't know of a short cut, but when I have to type words that aren't in the dictionary what I do is go letter by letter and hit the space key or the delete key. for example-if I'm trying to type "frackernack" I'll type the first two letters, find them in the list then hit the space key and go on until the right word pops up. not quite as fast as using a word in the dictionary but it gets the job done.