What's the lowest safe cadence on a climb?
Mountain bikers regularly run these low cadences for very short periods, often at much higher power output. The issue with causing damage to knees is more about the duration of the climb and how strong your stabiliser muscles are. (Too much time mushing causes chronic overuse problems, while weak stabiliser muscles can allow injury to happen), however they also have much lower gearing for sustained steep climbs.

If this is one or two short (2-5 minute) pushes over a longer ride, I would not worry about it, but be very careful about technique. If it's a couple of 15 minute periods over an hour long ride, I would be starting to look at options before problems start. 

Options to consider would be changing the route or going to lower gearing – If this is a regular route, putting a triple on the front (with a 24), would get your cadence up to about 50-60, which is ideal. 

If these are not an option, getting off and walking is not a bad thing. Don't let you ego get in the way of doing the sensible thing. You're already down to about 5-6km/h, walking won't be much slower, and as it uses different muscles than riding you are resting you riding muscles. 

Also be aware that if you are lowering cadence preserve aerobic endurance, you are putting less force on your knees than if you increased cadence - so without changing the bike set-up you cannot fix the problem. The mantra of high cadence presumes you can go down to a lower gear and hence reduce the force on you knees.