Doubtful. Asterisk can take quite a lot of cpu power so there is a possibility that it won't work properly.
But since the average $50 router can vary greatly in terms of cpu power, the only way to be sure is to try it. Asterisk has been known to run on cpu's as slow as 100 MHz Pentiums which is slower than what a WRT54GL has.
PS: to help others in the future you should mark answers as accepted if they work for you. Some people might not answer your questions with a 0% accept rate like you have now.