If I get this wall smooth-rendered ...

[ridikulos]

... does anyone know if I can reasonably ask them to leave the corner brickwork feature as is, or will they have to render over it? If left as is, do they use "masking tape" or equivalent to make sure it has a smart finish? (Sorry if this seems like a naive question, but I want to be armed with appropriate expectations and vocabulary before I get anyone to quote for the work).

Why would you remove an original feature and render bland, smooth 2020s style and take away all the period character?
A lot of people seem to dislike pebble dash, but it's exactly what a house the age of yours should be finished in.

They can render up to the brick. I've had my house fully rendered. They put up a beading system first which keeps the edges neat and they didn't go over it onto the bits that weren't to be rendered.