I'd be worried about the jury-rigged straps loosening, dropping camera, possibly into my front wheel/spokes, sudden deceleration, loss of teeth/skin, etc.

More likely you'd just bust the camera. Even if the straps hold, I'd be worried about it getting beat-up from road shock, rattling against bar, HT, etc, and possibly interfering with controls. Also, many bags claim waterproofness, but few actually are WP.

Probably best bet is to stick the DSLR in the Lowepro inside a pannier or rack-rop bag.

I don't know any way to easily, securely attach the Lowepro bag to a drop bar. This bag is about the same size as most dedicated handlebar bags. To fix to a handlebar would require a rigid Al panel fixed to the bag for support/mounting, actual hbar mounts, and presumably some way to remove bag from hbar to carry.

An entirely different bag might be a better idea. Take a look at the Arkel Big Bag, it may hold your camera with lens attached, maybe even another lens.

Few tourists carry DSLRs due to size, weight, fear of damage and/or theft.