Hi

Not sure if there's a way to solve this:

I have this combinator set up for outposts, hooked up to an LTN station to request items when chests run dry.

When I have items in my trash (i.e. wood, stone) they appear as "unfulfilled requests" in the logistic system, and so end up being requested and sent on a train.

Is there a way of filtering these specific requests or is this behaviour an engine limitation?

I cannot reproduce this issue. When there are trash items, they do not show up in any of the combinators added by this mod.
Would have to be more precise on your setup, but from what you wrote when I put stuff in my trash and walk into a logistics network, they do not show up, even if I have more than I have trash slots for it.
The only thing that shows up are the missing requests from the regular stuff that bots should bring to me, and only on the player combinator.