Seriously, not only does that work, there are companies which will send the quote back and ask you to requote them with a maintenance contract if you don't do it the first time.
(And yep, 20% is indeed quite standard at many places.)
And if it's separate, in what sense does enterprise purchasing use "maintenance"? Software updates?
90% of the time, they don't. It's a checklist item. They don't care if it ever gets used.
My last project was heavily reliant on a particular feature in a game development framework. We were more than 1 million into development when they announced they were discontinuing the feature. We had a service contract, but not a maintenance contract. They basically told us at that time that all those bugs we had filed over the last 6 months? Here's the code, fix them yourself. We pivoted as hard as we could, but it was too late and we had spent too much money. The end result is my company canceled the project and laid off my entire studio.
AR quotes maintenance purely as "We promise this product will be under active development for the duration of your maintenance contract. We might decide to do that independently of this contract, but if you want a legally enforceable promise which you can quote to other stakeholders, that costs $PRICE."