If you're playing for implied odds, how do you do that when you don't know who's going to call you behind you?

I think it's more +EV to limp in position then out of it, which is closer to the OP's question (not just open limping in general). Another reason not to do it much is habit, if you're doing it now you'll be more willing to do it when you move up in stakes, where it isn't likely to be allowed as much. Something to think about anyways.

1. If we take a quick mathematical look using 10 hands......
10 x .1= 1.00 spent
if half the time we're folding to a raise -> 10 x 0.1= 0.5
so we're -0.5 so far
using PT stats to generalize, let's say we go to showdown 30% of the time, and wine 50% of those showdowns.
we lose 0.2 (the time we're seeing a flop but don't go to showdown)
so that leaves 3 times we've limped preflop in which we would have to make back that pre-flop dollar. of course, this is simplified, so it doesn't take into account betting amounts invested on later streets. But the quickest way to get that money is to bet the flop, but how often are you going to bet the flop oop with a weak hand in a multi-way pot? i'm not sure, but you might be able to get an answer with pokerEV.
to see your performance of open limping utg with PT, do the following in the filter:
under VPIP, select put money in
under pre-flop raise, select no raise
since it's 6 max, select EXACTLY and 3 for positions off the button
(the only downside is this works only for sites that differentiate between 6-max and full ring. it won't work for sites that lump them all together.....someone will have to work on that and post maybe? )