Here's a really stupid work around I put together for the time being. It basically uses the equivalent of autohotkey or macros to enter the appropriate key presses to enter the BLF's. It's best if you have a separate computer to run this on as it will tie up your system while it's running.
I guess I can't attach this file type so I dropped it in pastebin,
https://pastebin.com/rAcdHWU7
Just copy the code, paste it in notepad or something like that and change the file extension to pmc. You also need the program to run it, it's free'ish?. It's called pulover's macro creator.
https://www.macrocreator.com/download/
I didn't find anything strange in the program or the installer but I make no promises. Anyway open your file in macro creator, line your cursor up on the first BLF field and run your script.
You might want to edit the file first unless you have exactly the same extensions I do, guessing probably not. Just change the Numpad# entries to your extensions.
This will populate the first 10 or however many fields there are by default on the BLF page then add another 10 or so then populate those. You should be able to expand it to however many BLF's you have or reduce it if need be.
Once it's finished it just stops, it doesn't hit OK or anything so you can review what it did. You can test it on a blank BLF page just to see how it works before you start using it. Be careful of starting this in the wrong window or anything like that. There's a lot of tab presses and things like that, if you run it on the wrong web page or some other program there's no telling what it could do and it can be tricky to stop. Again I make no promises or guarantees, use at your own risk.
Anyway like I said this is a pretty stupid solution but I can do other things while this runs. It's not fast in fact I can do it faster by hand but I had to slow it to this speed to get about 95% success out of it. The amount of time it takes for the extension number field to populate seems to vary quite a bit, at least when you are talking about milliseconds. Hope this helps somebody, have fun.
Thanks
Bronston