I will shortly be getting the chassis of 531 steam cleaned and silvered. Common sense tells me that prevention is better than cure, and that it may be prudent to protect certain areas before steam cleaning, as it will not be me doing it, and I will not be present. I intended to put polythene bags secured with plenty of Duck tape around the EP valve block, speedo generator and the wiring around the accumulators and pressure switch. Any thoughts on this, particularly from anyone has has done it?

My bus number (if any): RM531

I get to do high pressue cleaning most days, a safe way is to use the lance obliquely on areas where there are less hardy parts like wiring boxes, etc where the pressurised hot water can get in.

Many high pressure washers have a heater control so maybe prudent to set low, the one I use uses red diesel and runs at 135f max, I use it at 100f, but it is a question of what works.

On exterior panels again use the lance oblique to the panel and not too close, otherwise it can distort thin panels depending on the force used

Also the hot water can lift decals off especially vinyls, also some paints which don't have tough primer coats can be lifted off or stripped entirely by hot water, especially on aluminium panels where an etch primer coat hasn't been used

It really is just a case of common sense when using these machines

My bus number (if any): RML 2547

The HMG Chassis SIlver paint is pretty thin stuff so you can most likely use this out of the spray gun without the need for thinners.

A trick here if it doesn't spray easily unthinned is to put the full spray gun tank in some hot water, it makes the paint go on easier when the paint is hotter.

I have brush painted the HMG silver on some of the aluminium panels like kickplates around the floor edges and on the stair kickplates, it really makes them look like new. You have to put it on sparingly by brush and brush it out well, otherwise it can run being a very thin paint.

Acrylic spray lacquer in aerosol cans wil help to keep it fresh for longer, makes like Hi-Cote and Simoniz acrylic code UN1950 (acrylic) give good results

My bus number (if any): RML 2547

If you are going to steam clean and blast the frames and undersides then do mask off the areas of vulnerability as you suggested. It will save a lot of trouble in the long term.
As someone else once posted a lot of this in detail it might be either on ROOF or on the other channel.
It is prudent to do this in hot weather and preferebably on a hot but breezy day and then take the bus on a good run on a hot road. This 'blow dries' the underside out and reduces any oxidisation.

Then inspect and de rust any corroded areas and remove any flaking detrius.

LT only applied a good quality ally rich chassis silver paint but it was only expected to last 4-6 years max.
In preservation this paint does not last much longer so a better treatment can last 20 years plus easily.

The best thing then is to use an epoxy based underside coating, (Usually aluminium) It is unlikely you will ever need to deal with corrosion again as this is a great bonding formula almost impervious to the nothern atmosphere.
Ideal for preserved vehicles living outside or in damp conditions.

Failing that, paint the undeside in a zinc rich coating and then apply a aluminium rich protecting coating