When tapping single phase power from a 3 phase service, how much power can safely be drawn?
What you suggest is not advisable.

If you are not offered a neutral then you cannot assume that you have access to a 'Y' service.  As a 'delta' service you are able to use the phases for power.  Your phase voltage is also outside your control but over 400V.

The correct solution is to arrange a enclosed tapped 400-480V to 230V isolated transformer with a suitable receptacle for your instrument.  The ground will be passed through and connected to the transformer casing and to the secondary centre tap and neutral.  You will need suitable circuit breaker and over current and residual fault current switches.  This will eat into your budget a bit, if permanently mounted you will likely need facility electrician to fit and connect as well.

You can draw current to the maximum that your supply circuit protection devices will supply.  If your loading is heavy and always on the same phase (pairs) you may overload the phase(s) before you have used the full supply available.  Unevenly loading a 'delta' service causes one live to have less current, unevenly loading a 'Y' service causes one live line to have extra load and the neutral to carry undesirable but not a fault current.  If you have 50A service at 400+V you could have over 85A at 230V after your 20kVA transformer (this is a good size).

Facility electrician can probably wire a 230V outlet for less money than supplying and fitting a step down transformer with ancillaries.

If you had a 'Y' outlet you could use the neutral and one phase (live) to get your 260+V single phase supply at 50A.