Just spell out the rules in your NOR.
Handicap racing is conducted using SCHRS. Each skipper is responsible for registering with their proper rating. (reference table on web site)

If the owner cannot use the stock table or calculate their custom rating using the web site and available resources. The OA will generate a rating for the weekend using best available information and their judgement.

The OA is NOT measuring any equipment.

Designate somebody who understands the ratings process to serve on a protest committee in the event that you get a ratings protest from a competitor This is NOT the person who generates the weekend rating. . Let your judge / PC chair know the standards. They may want to spell out restrictions on equipment protests.... (that is a judge call there)

The stakes for the event are low and so you don't need high standards with certified rating certificates.... Its just overkill...

So for your situation.. by all means... Plan B
I think you have the conversation with them...
Non stock sails... OK... same size?.... or did you go bigger?

Same size different shape? ... use the stock rating.. Bigger... go with plan B. Nobody wants to cheat... they want to race once a year and enjoy the event. They certainly don't want to feel like they are putting one over on their friends in the fleet. So tell them... we try to be fair AND we lean to the fleet side of fair for your spitball rating . We are not measuring YOUR STUFF... They choose to register with that rating, get their stuff measured or go home. .... Tell them how it works... if they care enough... next time... they will bring a number and perhaps some math on a piece of paper. If you really want to accommodate them... allow them to submit a measurement rating before Sunday racing starts.... (ask the judge about this one)

If they are into the competition and boat design optimization thing... they will do the work themselves and present a number. They will have had plenty of time to get their rating sorted out when they follow your NOR rules.

I go into these things believing that people want a fair game with minimal stress. In the SCHRS world... the responsibility has shifted to the owner here. Your OA is just backstopping them so they can go racing.

I will try to get together with Will and create some NOR boilerplate that you can use.

Mark