I presume you mean an existing, erected stack.
There will be some, limited access from the ladders.
Many flares have a painter's trolley rail mounted below or around the top platform which allows a painter to sit in a bosun's chair and move around the stack. If your's doesn't, you could possibly fit one around the top platform which of course will need a BIG crane to reach the top.
More likely, if you discuss it with a painting contractor (and you should), you'll discover a preference to just use a bucket supported by the same crane.
A few years ago I used a system by Spider Staging
. It was a self contained, powered bucket which climbs a rope attached to a bracket at the top platform. Have your structural engineer look at the top platform brackets, which should be strong enough to support about 1000 lbf each at the outer end. Perhaps you can secure a moveable platform like this in different orientations to give a number of clear vertical runs.
Obviously, the configuration of pipes and the stack support style will have a lot to do with your decision.
![[ponder] [ponder] [ponder]](/data/assets/smilies/ponder.gif)
David