Nik_2213 said:

Does molten magnesium react with pure silica, SiO2, as powder or glass-fibre ?

Silicon is usually refined by heating quartz sand with carbon, in an electric arc furnace, to remove oxygen. Your question may become, can the carbon be replaced by magnesium ?
That is a chemical process called magnesiothermic reduction, which reduces release of CO₂. Magnesium powder reduces silicon dioxide to form elemental silicon and magnesium oxide, MgO, which is valuable as a high temperature electrical insulator.

Does molten Mg have sufficient energy to heat the glass to reaction temperature, or is further energy needed. It should be possible to retain the heat, to maintain the reaction. You will also need to keep atmospheric oxygen away from that high temperature reaction, or the Mg would burn in air to produce MgO + SiO₂.
https://en.wikipedia.org/wiki/Hydrogen_assisted_magnesiothermic_reduction

Nik_2213 said:

If conditions less extreme than an electric arc-furnace, could micronised silica be dispersed into molten magnesium without reaction ?

No. Magnesium metal is highly reactive. Molten magnesium metal will react with silica to produce silicon metal and the ceramic magnesia, MgO. You could use the ceramic for its environmental and high temperature stability.
https://www.preciseceramic.com/blog/magnesium-oxide-ceramic-materials-an-overview.html

Nik_2213 said:

Tangential, just as some ChemEng systems have 'Silica on S/Steel' plumbing & tankage for strength plus corrosion resistance, would 'Silica on Magnesium' be viable ?

Silica on Mg would not be stable, producing Si and MgO. Silicon would form an alloy with magnesium in the surface, with a coating of MgO on the surface. The eutectic alloy would probably crystallise as magnesium silicide (Mg₂Si). On either side of the eutectic there would be crystals of Mg or Si metal, depending on which one was in excess.
https://www.researchgate.net/figure...en-reproduced-from-ref-11-with_fig1_344552933

https://en.wikipedia.org/wiki/Magnesium_silicide
Mg₂Si is a low-density alloy with a specific gravity about twice that of water. Its melting point is about 1000°C. You could grow single crystals and try to make turbine blades. Mg₂Si can also form ternary alloys with aluminium.

Mg₂Si is a semiconductor so it could be used for heating and cooling, or power generation. It has a band gap voltage about half that of silicon.

You might make a Mg metal device, and then react the surface with an oxide to generate a protective capsule of ceramic, MgO.

Nik_2213 said:

So, rather than coat the magnesium with silica glass, Plan_B is a TBA polymer...

You can glaze the magnesium object, but only if it first has a surface covering of MgO, to isolate the Mg metal from the surface glass.