If I get my wall smooth-rendered ...

[ridikulos]

... can I reasonably ask them to leave this corner brickwork feature as it as is, or will they have to render over it? If so, do they use "masking tape" or equivalent? How will they make sure it looks smart? (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).

If the wall surface and the bricks are at more or less the same level, and render is applied, which is an extra layer, then there would be a drop in level where the render abuts the bricks.

I don't know anything about render, but I do know that tapering off a cement finish to get the level correct with mortar wouldn't work, because the tapered thin bit of mortar would be too thin to have structural integrity, and would tend to get flaked off by the freeze/thaw effect in winter.

Have you considered painting the pebbledash surface? Ok, it still won't be smooth, but it will look quite different, and the lovely brickwork would be highlighted. You could do a section, and live with it for a bit before making a decision. Although I'd check with a builder first that a painted surface wouldn't be harder to subsequently render over.