Adhoc/Custom One-Off Plans - possible?

Is it possible to define a one-off custom plan that isn't available to customers and assign that to someone?

The example might be that my application offers plans around an attribute and seats. For example:

Bronze: 1 seat, 1 website Silver: 3 seats, 3 websites Gold: 10 seats, 10 websites

'websites' is defined in the attributes of a plan.

That all works great.

What if someone enquires and says they want a custom plan, let's say for 25 websites? I know I can create a plan within my SparkServiceProvider that is archived() which will prevent it from showing in the application for customers to choose, but then how would someone sign up to it?

I'm trying to find an elegant way to do this. I could potentially show it on the frontend (don't archive it), then archive it once it's been signed up to, but that's not ideal. I'm sure this isn't an unusual use-case so wondering how others have done it.

Thanks