I understand it is probably more trouble than it is worth, but I am keen on using my 4863 IBM monitor as a second display for my x230. So far I've looked into converting its VGA signal to a CGA output, then using the CGA adapter for the 4863, but not sure how feasible this is as a solution. Any help?
Not very feasible.
You would first have to capture that VGA signal - how exactly depends on how do you want the result to look like, but either with a real video ADC or just some comparators to turn analog RGB signals into three RGB on/off bits.
The second problem is that the CGA monitor only shows a 60 Hz signal with 200 active lines at 15 kHz. You likely can't make the modern video adapter to output CGA resolutions and timings. So for getting any VGA signal to CGA timings, you need a video processing or a scaling chip.
It is unclear also how this can be useful, as VGA outputs have 16 million colours and the CGA only has 16, so the result would be pretty unusable. Unless you again use video processing magic and use temporal dithering, similarly how 24-bit (8-bit) colours are converted to 18-bit (6-bit) TFT panels.