What's the lowest safe cadence on a climb? I recently installed a power meter and just used it to get up some rather steep 15%+ climbs.

At my weight and lowest gearing (34f 30r) at a sustainable power level for my fitness (225w) this translated to a seated 35-40rpm cadence. 

Higher cadence would result in heart rates of 190bpm+ which I couldn't sustain over the climb without stopping. 

This is MUCH lower than what I usually spin at (95-100) and while I made it up the hills, I am concerned that this could cause damage to my knees/body if I continue to do this without losing weight or increasing power. (Working on this :) )

I have searched, but there doesn't seem to be much on this topic. 

A triple is not an option as I am doing this for fitness/fun and in the long run the compact I just upgraded to (50/34 from 46/36) will be enough once I get more KM into my legs.

These climbs were 1-2km in length with up to 130m elevation gain (Cat 4), but the question is more about doing this for extended amounts IE (Cat 1 / HC)

Standing for that long of a climb is not achievable for me at the current fitness level. 

I didn't feel any discomfort in my knees, but everything I have read suggests high cadence cycling but I am not there fitness wise yet on these climbs.
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.