Unfortunately, the second option is somewhat not possible, since there's no way I can "define" an output to be a script only output. At least, it wouldn't visibly appear to be script-only.
However, as to the first suggestion, I should have some code that does that exact thing, pushing things that can't be recycled into the output slot. I would have to ask if perhaps you have that option disabled in the settings?
I'll take a look for myself, but I'm pretty sure it was working the last time I checked. Something might have changed in Factorio that has broken this code though. Or I just didn't check as properly as I should have. Either way.
EDIT: Sorry about not following up on this, but I recently got a job, and had to do a lot of back and forth over the last week or so. I should have some free time this weekend to take a look at this issue.