In reply to sbattams:
There seem to be two separate issues up for discussion here:
Is it OK to use a non-specified marker pen to middle-mark my rope?
The answer will always be no, because a rope manufacturer cannot test every product on the market to see if it damages the rope. What happens if a product is tested as safe, but then the formulation changes without them knowing? It's pretty unlikely that a small amount of ink on the sheath will cause a rope to fail, but......take the risk yourself by all means but don't expect anyone else to stick their neck out and say "sure, it'll be fine!"
About the Beal rope marker tests - firstly, I'm not sure if the tests done were done using the UIAA sharp edge test protocol, but that protocol is defunct now because the results were too variable. In other words, it was an unreliable test and I wouldn't set any great store on it. Also, there was a rumour going around that in the tests, so much ink was impregnated into the rope that it lost much of its elasticity on a local level. What I do know is that all dynamic ropes are made from pretty much chemically identical raw materials, so if something is safe for one rope, it should also be safe on another. Again, due to the "not sticking one's neck out" principle, you'll only get a manufacturer saying the marker is safe on products they have tested, because they won't know if another brand decide to use a new material without telling them.
Before anyone complains about manufacturers being overly nesh about these things, remember that messing it up can potentially mean jail and/or the closure of a business supporting many families. C'est la vie!
In summary, use an authorised rope marker if your rope brand allows it. If your brand says no, a rope marker is probably less risky than any old marker pen but this is at your own risk and of course cannot be recommended.