Reddit mentions of Control Tech Terminator MTB Handlebar Extension Plug, Black

I have a couple ideas:

First, see if there are adaptive bike resources in your area. If not, you could email some in other areas. They'll probably know of systems that have been tested and work well for riders needing a similar accommodation. Maybe these people would have some ideas: http://www.notbroken.co.uk/2016/06/15/bike-check-tom-wheelers-mojo-nicolai-geometron/

Togs would create a thumb hook that could be used in combination with just about any setup



I'd like to make a suggestion other than an expander plug or pipe plug, because it seems a little dangerous given that they are generally made to resist vertical loads (or loads in line with the fastener) like a cork. wiggling a cork sideways is a good way to loosen it, and that's essentially what would be happening every time the pedal is pushed or pulled. I also think it's going to be hard to find a proper expansion plug that will accept a 9/16ths threaded pedal (unless you're planning on replacing the axle with a custom bolt that serves both purposes).

However, if a nut was welded into the end of the bars, a pedal could simply screw in. You might want to consider a pair of steel bars that are on the heavier side with thicker walls, so there's more meat for welding. I could see the nut on a thin walled set of bars just cracking or ripping out since the force is going to be distributed differently than designed for.

you could try welding a nut into the end of something like this. But honestly, if a non permanent plug comes loose, your friend might be headed straight into the ground head first with no warning.