As has been said, the sheet metal rear sight being replaced lends itself more for custom sight, but it would require milling on the rear dovetail and possibly the front sight and reblue.....which can run into some bucks. You can sometimes find like new replacement rear sights like yours on Ebay and Gunbroker for less than $20 - but you dont sound like you like it.
Check with Midwest Gun Works to see if they are expecting anymore adjustable sights. There is a later model Browning adjustable rear sight that fits in the same dovetail, but it looks more like a smaller version of the MICRO sight often seen on the old IPSC combat comp pistols, shown below.
http://www.midwestgunworks.com/page/mgwi/prod/browning-hi-power-sights/B515554711
Otherwise find a friendly pistolsmith with some milling skills and serious machine tools.
PS - If you check the dovetail size, you may be able to cut down one of the tall suppressor sights offered for the Glock or SIG pistols. Only problem I can see is that front sight is pretty wide.
I used a BHP like yours for 20 years and grew to like it. Especially as my eyes have grow more farsighted. Good luck.
Here is a pic from another person's post about his modded BHP for some ideas. It is not my pistol, so I cant provide any other info - so you might try a picture search to find the original poster.