If your willing to take partials using market orders you can, just change LIMIT to MARKET and leave out the 'Price' bit, as here you don't need to specify the bid or ask, the script below for instance will partial 25% of your position whether you are long or short.
CXL ALLSYMB;ROUTE=MARKET;Share=Pos*.25;TIF=DAY+;Send=Reverse;
I'm very new to BBT and trading in general, so maybe I'm missing something, but it seems to work fine for me.
On a different note, thanks Kyle for your dollar risk hotkeys they've helped me alot.