You might equally well ask whether it is safe to supply a SP circuit from a TP DB; or what about for example, a lathe with a 3 phase motor which is not running, but a SP lamp and DRO which are.

I don't see a problem.  Graham's caveat concerning RCD protection is entirely reasonable, but even then, taking one phase off a 3 phase plug isn't any different from using a BS 1363 plug in a circuit which (due to age) has no RCD protection.

Perhaps the OPs fear is the commonly held mis-belief that taking a single phase load through a three phase RCD will lead to it tripping....

Perhaps the OPs fear is the commonly held mis-belief that taking a single phase load through a three phase RCD will lead to it tripping....