Not sure what you mean by bars not being standard size. On my HPV Spirit the diameter at the stem clamp is 25.4 mm, then is 22.2 mm elsewhere. Even the Rans Stratus XP with its extravagant chopper-esque bars stays with these dimensions. I manage to fit all my stuff on the Spirit without any problems.
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After being used to bikes with USS it's like going from a famine to a feast.
From left to right: bell, Garmin etrex, bottle cage (though there were already fittings on the stem), Cateye backup light mounting (bike came with dynamo hub and lights), and Mirrycle adapted to work with a handlebar clamp. These items have a lot of adjusment for different bar diameters though. I would think that the bracket that you linked to would work OK if you wanted to keep your bars clear. Something that I found useful on a previous bike was a Minoura Space Grip which I unfortunately sold with it. They don't seem to be available any more but there are a lot of other brackets about which could do the job.
If you look at the derailleur post there might be a threaded hole somewhere on it which might take a light bracket. It could have a bolt or rubber bung in it to protect the threads. You might be able to fit a light to the post itself using a clamp of some sort. My Grasshopper had a threaded hole hidden away under the boom (it didn't have a derailleur post) but some HP models have one on top. It's a bit random, seems to vary between models and year of production, from what I've read.
