Are there good ways of getting solar cells (PV) onto old buildings? So far, I’d have to say No—one gets gleaming rectangular grids on tiled or shingled roofs above weathered walls, and the result is a monstrous hybrid. Peace has yet to be achieved between the industrial look and the artisanal.

A solar panel is always going to be an industrial thing in itself. Silicon vapor deposition isn’t done by artisans; the craft and skill are industrial. But putting the panels into or onto the roof can have an artisanal element. I imagine some kind of framing that sits between panels, maybe projecting a little above them, so the plane of the roof isn’t uninterrupted and mirror-smooth. And maybe some decorative elements, like the crestline of the older glasshouses. Maybe even a change in layout (orientation? proportions?) at the ridgeline and/or eaves, to say visually that the panels know they are part of the particular roof. I thnk those things could be done without reworking the whole structure of support and waterproofing, the way skirting boards deal with the join between wall and floor. But not only at the very edges. There should be some modulation or punctuation of the surface.
I think this is mostly a problem of drawing, i.e. of trying things out visually. Well, it’s a problem of mentality, too. There’s an industrial mentality and an artisanal one, and they don’t readily go together. But we should remember that some industrial products fit artisanal work quite well. Bricks. Trim. And there’s an in-between realm of work where molds and templates govern the general shapes and forms, and handwork does the essential finishing.
PVs need framing. That’s the one-liner. And the framing needs to be suitable to the situation. It has a functional job to do, and it needs to have the right feeling. There must then be a variety of ways of doing it, just as there are varieties in the framing of windows and doors. Doors have doorways. Roofs already have ridgelines and bargeboards, not to mention chimneys, dormers and other interruptions. These can be modern or traditional, and each of those umbrella categories has all sort of variation, actual and possible, within them.