I have a hightop (brand unknown) on my conversion van. There are a few options that have super tall legs that will actually reach all the way to the stock gutter channel and mount to it as a typical roof rack. I don't recall the name of the company that makes these off the top of my head, but they are like $400 for two cross-bars if I remember correctly and they look goofy as hell (my neighbors used to have this setup).
Yakima also makes "artificial gutters
" that install in to the side of the roof and basically give you the ability to place roof gutters wherever you like that will allow you to mount normal cross-bars to them. I used these - they are sturdy (I used yakima A1 towers along with them). The benefit of these is that they are attached to the strongest part of the high top (the vertical section, rather than the horizontal section) and they allow for widely accepted round/square cross-bar setups made by Yakima & thule. I've run this setup for nearly 3yrs with no issue.
I am currently in-progress of transitioning to a unistrut setup that allows for a lot more adjustment in rack positioning - a great resource for this can be found here.
In terms of weight carrying capability, I believe fiberine offers some reinforcement options on their tops that will help with this (in the form of wooden reinforcement on the interior of the roof box). I've stepped on the outer edges of my hightop while accessing my roof box & solar panel with no issues, but I'd never step in the center of it as I don't trust the strength of the fiberglass across the center (my roof to is a fiberglass/honeycomb cardboard construction, not as strong as fibergine tops, I believe). If you want the ability to walk/easily access your roof area, the best setup in my opinion is a stout roof rack that has metal or wooden slats that can be walked on (rather than trying to walk on the fiberglass to itself).