Is it possible to service rear derailleur pulleys? The outer pulley of my RD-M735, when spun, sounds scratchy and feels slightly grindy as if there is a tiny piece of dirt/grit that has fallen inside.

I have literally soaked it in WD-40 and then 3-in-1 oil and also, motorcycle chain oil (scottoiler oil), whilst spinning the pulley and it is a lot better than before. But it still catches now and again when turned by just finger tip, and sounds slightly dry and grindy. It will turn under two finger pressure though.

Therefore, I was wondering if the pulleys are serviceable, inasmuch that one can access, clean and repack with grease, the ball bearings, or  replace the bearing (that I presume) are/is within the axis of the pulley?
Yes, the pulleys of your rear derailleur can easily be removed, cleaned, greased, and re-lubricated. I'd recommend doing so about every 500 miles depending on your climate or whenever they start to sound gritty again.

If you haven't yet, I recommend reading all the Park Tool drivetrain care topics.

Steps 9 through 13 particularly on this page should cover the pulleys:

https://www.parktool.com/blog/repair-help/rear-derailleur-overhaul