yup, a simple framebuffer only needs 2Mbyte; 8Mbyte is a lot of video memory if your purpose is just to have a fb-console to show some basic graphics.
Anyway, Friday a local recycler offered an MS-9513 video card to me for the cost of a beer, and I got a chance to try it, even if I am just a bit confused about the reasons why it doesn't work at all.
A router like the Ubiquiti's RSP doesn't implement any PCI IO functionality, while the Miktronik's rb532 does implement it, hence a video card should have some chance.
the Mini PCI was introduced in version 2.2 of the PCI specification
hence it's a 3.3V-only bus
the Ati RAGE XL's chip is 5V tolerant
hence it should perfectly work in a 3.3V PCI bus
However, what I observed yesterday is too odds, because when the MS-9513 was installed in a 5V PCI system (Pentium4 PC, via miniPCI to PCI adapter) it was correctly revealed with the correct vendor ID and product ID, even if it didn't work and the Linux kernel immediately crashed after probing the card; whereas when MS-9513 was installed in a 3.3V PCI system (router rb532), the video was not even revealed as PCI device.
This makes me wonder if Ati Rage's chip in the MS-9513 card was somehow customized for IBM workstations since the MS-9513 was specifically made for being used for IBM's products.
Here is where things are confusing because the ATI Rage XL datasheet tells there are some hardware configuration pins, but they are unfortunately under the BGA chip, hence not directly accessible.