Trouble is, there may need to be almost a point
load of 250kg on it near the middle

Mmmm.... 150mm height, 15mm web, 250kg...
.... equivalent to 2 rather large people
.... equivalent to another largish RSJ sitting on it
.... equivalent to 0.3m wide crap upstairs block wall

I think the RSJ is likely to say "that all you got" :-)

The BCO is more interested in the bearings.
That is, crumbly crap mortar vs tightly packed slate or padstone.