Question: If possible, i'd like to run a find & remove query on non-indexed columns in "background", without disturbing other tasks or exhausting memory to the detriment of others.

For indexing, there is a background flag. Can the same be appended for find/remove tasks?

Thanks for a tip

Answer: 
This is not something you can use "background:true" for. Possibly the best way to handle this is to write a script that does this in the background. This script should run your operation in small batches with some delay in between. In pseudo code you would do:

* find 10 docs you need to update
* update those 10 docs
* sleep
* goto first step.

You will have to experiment with which value for sleep works. You do need to realize that all documents that you are updating need to be pulled into memory, so it will have at least some impact.
No, there is not a background:true flag for this operation. The remove will yield when page faults occur and allow other operations to execute. If you need to throttle this, then you can either remove in smaller batches or use a find/remove pattern which will lower the impact to other operations.