There are a few problems with those suggestions.
Anything earned as a consequence of playing rugby should be deemed part of the cap. So, in your example, that sponsored scrum cap would count.
But, you simply can't rely on players, the clubs, or the financial supporters of a club, not to make side deals. A relative gets a job, a house overseas is gifted, a holiday paid for.
In effect, all benefits in kind as understood by HMRC.
Yes, a player might have a coffee business, or own a pub, or ... they have to be allowed to develop those businesses as a retirement plan if nothing else. But, that then opens the way for money to be laundered to them though that business.
There is not be a way to stop it. Look at all the tax avoidance legislation HMRC have at their disposal, and all the investigators they employ. Along comes an upright citizen like Nadhim Zahawi. Walks all over it all.
No amount of oversight by the RFU, no team of investigators, is ever going to uncover the naughty stuff as it happens. You will find out years after the event. And the punishment for the high and mighty is never enough. Nadhim should have gone to prison. Cheats should have been thrown out completely. All their trophies taken back.
Right now I guarantee that Cheats are still cheating, and that at least two other sides are to a similar extent. It makes me wonder. Is it worth watching/following/supporting professional sport at all? I think I would feel better if Wasps never went back to the Premiership, and operated more like the Championship clubs. Leave the dishonest cheats in the Premiership, walled away in their self righteous glory.