Back in the seventies and eighties, the general rule on most every airline was that nobody from Coach was ever allowed up into the First Class cabin for any reason, be it using the lav or visiting with an acquaintance. This rule was applied regardless of whether the cart was in the aisle or not.

This rule was strictly enforced on all airlines (the curtain was almost always closed back then) and what's interesting is that for the most part, nobody from Coach ever complained about this. It was seemingly understood that because they had paid less money for their tickets, they had to accept the longer waits - even if the carts were in the aisle.

Perhaps the current more relaxed approach is the inevitable result of years of free upgrades combined with the ongoing homogenization of the domestic air travel experience.