You can, it is just a much, much tighter fight and requires you to move tighter.
That interesting, if you have something I am interested.
My mains problem if I try to swim inside (or swim through) the punches is that I can not really avoid a MT type of clinch. I mean it is not so much that you come in the middle but because ukes usually want to avoid elbows on your way in and get in better position to avoid the knee in the head. Uke tends to pre-empt a possible clinch by "initiating" it, if you see what I mean.
Other than doing the step on the side to counter that (hand getting in the same generic case as a slip), I can not really prevent him to close.
That being said I really dislike coming in the middle like that with my arms coming from the outside. I prefer the stepping on the side/slip thing, being inside or outside uke arm does not really matter.
Just to make sure that is what I refer to by swimming inside http://www.youtube.com/watch?v=GAo8YNxBwFc
The baddy does the swimming in that video