I've never done this personally and my only practical experience with it is when I was helping tune a Pro 5.0 Racer who had to run a spec lift Hydraulic Roller (like a B303) cam for the class.

They would run the Morel Solid roller Lifter because back then at least, maybe still Morel used the same long Hydraulic lifter body with a Slug inside of it instead of a plunger. They would set the lash tight like .004-.006 (had to run factory iron heads) and they'ed pick up 500-600 RPM and maybe 30-40 more HP

For Flat tappets I would think the faster ramps (and usually stiffer springs) of a solid Cam would make that a 100% no-no.

I have heard of poeple running "near Zero" like .006" lash solid rollers on Hydraulic roller cams...I would think that would essentially be just like having a milder off-the-shelf solid, So maybe if your HR combo was a bit Over-cammed, that might actually be doable. I would think the lighter bodies (like the BAMS) might offset some of the rotating Mass and with a smoother lobe, maybe that's a thing? Has anyone actually done this? Like I said, I have no skin in that game.

But I cant imagine going hydraulic lifter on an aggressive Solid ramp. After all Hydraulics are "rate of lift" limited by design, that's why they still make solids after all...or so I thought.