I'm just starting to add the vertical strip pieces for the cockpit coaming and it occurred to me that as an extension of the rear side pieces downward I could incorporate hip braces as part of the coaming. Has anyone tried this and what are the negative aspects you found?

I haven't done this, but I've made a few boats with 'side plates'. I think the Pygmy boats use plywood side plates IIRC. (?).

For me, it would be tricky to glass the faces and edges of an intgegral strip 'side plate' vs installing a pre-epoxied & glassed side plate. Especially with strips, I think glass overlay would be important to support backband bolts, if that's the system you are using. So, for me, glassing a (plywood) panel and installing it later would be easier

A bit off-topic:
There are definitely two 'schools of thought' on side padding in the seat area a.k.a. 'hip bracing and padding'. One idea is the 'your body should pop in and out of the boat like a cork in a bottle and not slide around' and the other is that it's an advantage to be able so slide to the side when you are rolling, balance bracing, rotating your hips for power in the forward stroke, etc..
The 'no side padding' idea tends to be favoured by Greenland-style paddlers and rollers but also by some well-known 'standard style' paddlers like Shawna Franklin. It was Shawna who suggested to me that removing the foam side pads from the NDK (Romany) seat would be a good idea.