I cannot find anything in the code that will not allow electrical conduit supports to be attached to supports of other trades. e.g Steam fittters pipe , plumbing, sheet metal work etc.
If there were such a prohibition, I would expect to see it in 300.11. But it's not there. I think that as long as the conduit is "securely fastened," as required by 300.11(A), it does not matter what it is fastened to.
Problem being what keeps other trades from supporting off our support system and vice versa. I am sure it must be able to support the weight therefore according to 300.11 b raceway supports must be identified for that purpose!!
The way I read 300.11 I can hang a 2" conduit off of a 3/4" gas line (so long as it is secured well) but I can't hang a 1/2" emt from a 4" rigid conduit?
I have worked on many jobs where trades shared allthread supported unistrut trapeze, either split half and half parallel or stacked in layers. I think it is a bad idea to support your work directly from someone elses pipes or tinwork (unless it directly serves that piece of equipment) because they may need to make changes to thier system and odds are they would cut your work free and let it hang poorly supported or not supported at all.