Off the top of my head, if I wanted to disable the surge brake but keep the parking brake, I'd drill a hole thru the tongue assembly for a bolt that would prevent the inner tube from sliding in the outer tube. Then have some rubber inserts to seal the holes if you wanted to remove the bolt to use the surge brake again. You could drill it off center so as not to damage the damper/spring or else drill behind the rear mount for said spring. Or just add a stop of some kind inside the tube to prevent the inner section from compressing under braking loads Early trailers had a lever under the tongue that you could flip up to prevent the tongue from compressing whenever you had to reverse the trailer so that approach might work. I haven't looked at my trailer so there may be a better way. The common thread is that the pparking brake handle actuates the brakes thru the brake rod but independent of the sliding tongue.

Chip Bearden