I fully appreciate the desire to make a Dexed for the fun of it, however .. if you haven't dug into Zynthian yet, it won't be obviously clear that:

>Boss SY synth pedal or something signal-processing-based like it

.. is exactly what the Zynthian delivers, plus way, way, way more. You can run Dexed on it, and also run multiple signal-processing chains for your lap steel. It is huge bang for the buck! Especially if you do live things, you can have Dexed tracks running in parallel to your FX chain ..

Just sayin', take a closer look. For lap steel (and indeed any similar instruments), Zynthian is a godsend. (Get a good mic for it too, of course..)

I am positive you will find it extremely rewarding to pipe your lap steel through these:

https://zynthian.org/engines#effect

.. alongside these:

https://zynthian.org/engines#synth

(Dexed is but one in a very, very sexy list..)