[Orange]

It's the same booster Volvo used (ATE), as the ones on Mercedes. ATE is an european part maker so they put these parts in several european cars (I think in VW as well).
If the pump runs a lot, it looks like there is a vacuum leak indeed. May also look where the hose enters the booster (engine bay). See perhaps if you notice black smear on that white tube (need to remove the grey 'accordion' cover first. May also follow all hoses from the switch, to the intake, pump, see if everything is tight. Also have a look at the pedal sensor switch at the booster (the black cylinder attached to the booster inside the engine bay), it has a seal to it.
As for the job, I would try replacing the whole booster on a XC90 - because the seal only job can be very tricky. There should be enough space to remove the booster and you know the whole unit is new. Careful with the tricky switch at the brake pedal - I think I have a few videos with these repairs on S60

 

[Orange]

Yes, I think this would be a good maintenance item to lube the seal and tube with silicone grease. Long interval to be sure; but I wonder if the seal failure is due to the grease eventually drying out.

I too believe the failure mode is rather the rubber itself getting softer and softer as the years are passing by. Not long lasting quality. Several european part brands work this way, designing parts to last around 10 years (may think at the Behr cabin blowers or the control arms bushings along others). Perhaps some silicone will help extending a little the life of the seal, but anyway once it gets soft, there is no way back. I had the new seal and the old one side by side, the new one is so stiff, if feels like hard plastic compered to the old, melted one.