I want to keep run payouts low to keep the game street level and gritty feeling. And to ensure that there's actually some punk in the game along with all the cyber.
But I don't want to unfairly penalise those PC's that are gear dependent like street samurai.

So I'm thinking of allowing karma to be spent for the buying of gear.

I'll probably use the karma to nuyen ratio from the karma character creation system in Runners Companion. And I'll likely also require some amount of downtime, exact amount determined by an extended test similar to how current rules for buying skills and so forth work.
In universe this would represent a successful, consequence free offscreen acquiring of gear through theft or some other process that doesn't change a PC's nuyen total.

PC's would of course be free to buy gear with actual nuyen, organise runs to acquire kit, and so forth. The new system would exist as an additional way to get big ticket items like cyberware while struggling to make rent.

Has anyone implemented something like this before?
Any suggestions to improve things or potential pitfalls that need considering?

Sounds just like Karma for Nuyen, and the usual gear acquisition tests with extra steps, to be honest.

You're abstracting an already existing abstract system, and adding another bunch of rolls to it doesn't exactly create atmosphere in my imagination.
Acquiring the gear with the usual rolls, and skipping the Karma to Nuyen part to directly put it into the gear cost might be much easier, but I doubt it creates gritty feelings either.

You might consider using some of the karma to acquire a contact who can provide the gear.

Easiest way to accomplish this might be to establish a Spirit Pact with a Free Task Spirit.

It wants karma and you have it.

It has the Wealth Power and the Skill to construct whatever you want.

On a regular basis, you meet your otherworldly patron for a spooky ritual which it uses to siphon off surplus good karma, while handing over the equipment it has fabricated since last time. Everyone is happy.