I have not done it; there's a good reason to not do it.
I'm not sure I'd characterize it as unsafe, but it is certainly unwise. Brass frame bp revolvers are known to suffer damage when subjected to repeated heavy loads; this damage is manifested in deformation of the recoil shield and cylinder support structure. Eventually the cylinder/barrel gap opens up, the cylinder end play increases, the action parts become loose, etc. Pretty much all the critical dimensions are changed. In the limit, if the abuse is continued the gun will become unsafe due to the inability of the cylinder to achieve proper timing.
As I said, this is due to repeated use of heavy loads. BP cartridge loads create less pressure than smokeless powder loads, and so are thought of as 'weak' in comparison. However, they are typically near the heavy range when compared with loose black powder loads, so they would be capable of producing damage on a brass framed revolver.
All the manufacturers of conversion cartridges warn against using them in brass framed guns. While one might argue that safety is not an issue for a couple of shots, it is certainly not a wise thing to do more than a few times. And if you can't do it repeatedly, why do it at all? Why intentionally damage your guns?