Yes, I've considered velcro e.g. Accumold. On what I've seen, it looks like the extra thickness of velcro isn't much better than the buckle I have. It's comfortably worn to the side in the natural dent where the oblique muscles join the abs. (or between the beer belly and the love handles, YMMV )Have you checked out the velcro 'underbelts' ...?
The one I use is only 3/32" thick by 1&3/4" wide. The duty under belts I know of seem to be as thick or thicker than a regular civilian dress belt 1&1/2" wide. A search under "duty inner belt velcro" will give you many examples.How thick are these underbelts?