I almost always hear 来 pronounced as lài in 回来了, but are there any rules of tone sandhi that account for this?
I am a native speaker. The term 回来了 is of course pronounced with ㄏㄨㄟˊ ㄌㄞˊ ㄌㄜ˙, so the second character is pronounced with 2nd tone. And the word 来 can only be pronounced with 2nd tone. (I've never heard 4th tone.)
Single character will have generally accepted tone or tones，but when used in combination with the others，things may get more complex. It is even better for you if you can remember which tones are not commonly used instead of the “correct” for a character in a phrase. In this case，I would say the third tone is not quite common，but you may hear the rest from time to time as they kinda show different emotions or hidden intentions.