If your requirements get close to "normal", whatever that might be, chances to get what you want increase.
You might get something in extra, but that might not be a trouble usually.

An example of this is the PC in its many incarnations - although it fits a different class of applications than discussed on this forum. I admit that in embedded the "normal" "class of applications" has a much wider meaning than in the PC-world.

Some three years ago I was using for a couple of purposes almost exactly the setup Eric was asking for. A small board, equipped with a uPSD3212 (a dual-UART 12-clocker), one UART went through a MAX232 (including RTS/CTS handshake lines), the other through an isolated RS485, with various on-board options (picked at time of assembly) for powering the isolated side. The uPSDs are natively ISP'd through JTAG, with all the consequences of this. As they are also IAP-able, I have developed an UART-bootloader for comfortable in-field update and a couple of other utilities. There was almost nothing else on the board, except some digital inputs/outputs circuitry and a FRAM for comfortable data logging (that could of course be not populated). I think this would fit perfectly Erik's needs. Whether I could persuade the company I was working for at that time to sell Erik some (plus the trouble of getting them across borders etc.) are all a different issue. But at the same time, we would be more than happy if we could buy those boards off-the-shelf, rather than waste time and energy for building them.

One of the boxes I am working on today is... an RS485/RS232 protocol converter enclosed in a DIN-rail box in a rather industrial style. That again might be meeting Eric's needs, except that it is not '51 based... ;-)

JW