While it's novel, there would be very very few 90s Subarus still on the road that didn't either have zero need for reflashing, or already having an aftermarket ECU, given the excellent availability and affordability of things like Links.

The 90s Subaru ecu has a 16bit mistsubishi processor but Subaru used an uncommon rom chip(made by Fujitsu) and in most cases didn’t even populate it. There is a small jumper pad connection to cut that enables the rom socket if needed. The jumper is designed to be cut and resoldered if it needs to be connected again. It’s a 16 bit rom so should have 16 pins for input and 16 for output which is a common chip like 27c256 with 40 pins. To save space Subaru used a 28 pin chip and the input and output share pins. There have been boards that run dual 40psi 27c256 chips for decades so dual 40 pins usb ROMs will work but you’d the adaptor as well.    There have been a lot of recent developments USB flashable ROMs mainly for retro computers and some cheap and fast 40 pin versions are coming soon. They hold up to 8 (maybe 16)different version so you can swap between them with a little dial. If you add a switch to the jumper you can have the internal factory rom as well as a fall back. The ROMs are programmable over usb from a web browser, it may support android so you can load ROMs from your phone via chrome.     A single option maybe possible if the developer will support the Subaru chip variant so you need a single rom.  If a few people (I know only one person now with a stock 90s ecu) are is interested I could ask the developer to see if they make a Subaru version. I have found all the data sheets for the Subaru ROMs so they can see if it is possible. They would list them on their own site and eBay(charges the -$8-14 gst for customs automatically).   Running an old dual rom board should be possible with 2 modern usb ROMs. At nzd$50 each rom plus postage and a boards it maybe a cheap option rather than aftermarket ecu.  If not already available I could upload a PCB to PCBway and jlcpcb as a community project and people can order the board themselves. I think it’s about nzd$20 for those shipped to on the cheapest 14 day shipping.    You could have multiple ROMs for road, track, valet and different fuels. Plus if you run a usb cable out to  the dash reflash without removing the ecu.

we usually run links ecus, G4X, if it is a 08+ it will run the stock Subaru can bus and the link one (that you can customize). A few emtrons in the same way.

Tweezers are fine since they heat both sides at once, and won’t heat the plastic connectors nearby.     But the fact it’s only a few parts to get canbus support on those old ecus is pretty good.  Apparently link may have done it for no cost (other than shipping maybe) when the G4 firmware was released. It has some limitations like it doesn’t support canbus for wideband 

Man air soldering or nothing when it comes to SMC. Especially small ones. Way too easy to f up the board or lift a trace.