"Spare" cast iron table saw top ... (in my hallway)

[julianf]

You'll need a fence if you ever want to machine a T-slot, for example.
(I'd've said a dado/groove, but you have a table saw for that!)
Having said that, a fence for a router table is a doddle: bolt at one one, clamp at the other.

As to the table:
I'd mill out a rectangular hole that an aluminium [clear perspex has its advantages] plate will drop into.
Bolt some 6mm thick bar around the perimeter (underneath), with grub screws to support the plate, and you can microadjust it so that it's level with the top.

The aluminium plate is bolted to the router in place of it's base and the router can then be lifted out and used freehand - or just adjusted.

There are those fences where the indeed and outfeed are adjustable, like a planer table.

So, effectively, small pieces can be, umm, planed.

Would I ever have need of that arrangement?

I can't see that I would but I've never used a router table before.