Here's how we do it. I can take pictures tomorrow if it'd help.
Sydney's currently rear facing in a Marathon/Decathlon. We have never had an issue with the straps interfering with the g-tube. I was all prepared to tuck some soft cushioning around the area when she first got it, but honestly - she was discharged about 24 hours after getting her tube placed and it didn't bother her at all to have us tighten the straps.
What kind of pump are you getting? I HIGHLY recommend the Zevex Infinity pump, it is very light and IME so much easier for them to carry around than even the Kangaroo. We have an Infinity and we just buckle her in like normal with her extension hooked up (you'll use extensions no matter what kind of pump you have if you have a button, these are the 12-24" pieces that go from the button to the actual pump bag) and we run the extension down the side of the car seat. We then hook her pump backpack using a carabiner to the headrest of the seat in front of her (since she's rear facing) and the extension connects to the bag with the formula that is coming out of her pump.
When she's forward facing (which she is, very occasionally) - in DH's sedan we buckle her pump backpack into the seat next to her. In my minivan we attach the pump backpack to the headrest of the seat her car seat is in, so that it is completely behind the headrest. If you were really paranoid you could buckle the pump backpack into the very back seat.
I honestly find FFing MUCH easier in DH's car and RFing easier in my car for tube feeding, but neither are so much easier or harder that it makes me go, "oh, we HAVE to install her car seat that way" if that makes any sense. I'm not dreading turning her FFing by any means in my car.
OK, pictures of some of what I'm talking about
LMK if you want pictures of Sydney strapped in with her tube attached. It might help you picture more what I'm saying.
there's a button - that one shown is a Mickey button. They are probably the most common. We personally use an AMT Mini One button, which is a different brand/company. We like AMT better because they are lower profile and they seem to last longer. The Mini Ones take the same extensions as the Mickey buttons.
Those are the extensions I'm talking about - those happen to be AMT ones, but Mickey ones look about the same. You will want the right angle extensions like the one on the right.
Then
- see that red port at the end? Those will then plug into the top of the extensions. The blue part loops into the pump, which if you are using those Zevex bags would be the Zevex pump, which looks like the pump on
this page (sorry, I couldn't find a picture)
and then fits into a backpack like this:
The other pump that is very popular is the
Kangaroo Joey. Most people I've met who have tried both hands down LOVE the Infinity over the Joey, especially with smaller kids, as the Joey is twice the size of the Infinity.
Also, another thing to remember is that in an accident, your biggest worry is the pump being a projectile which is why you want to at minimum carabine it and possibly buckle it. I honestly can't think of a way, if you have the pump fastened down, that it could move enough to pull her button out of her stomach. And if it somehow did, the button will just come out, it's not hugely traumatic. I have only pulled Sydney's button out once accidentally...by stepping on her extension and then trying to pick her up
She was totally fine (once she got over the initial shock of me pulling out her button, lol) and that was WAY more force than an accident would have, if that makes sense? Since her backpack cannot fly very far in an accident if fastened to the headrest or at all if buckled in.