While working on X-Wing 2.1.2, I decided that I wanted to make a custom icon for it that would work withDoorsCS and MirageOS.

There was basically no info on the mirage OS side of things- Detached Solutions's website and the MirageOS Developer's manual never actually go into the creation of icons for MirageOS.

After an hour of trying and reading through the DCS7 sdk manual, I've come to the conclusion Basic programs cannot have 16x16 icons(which consist of uncompiled ASM) in MirageOS - only ASM programs can.

DCS7 works fine with these 16x16 icons in BASIC though, which was the source of my confusion.

TL;DR hybrid MOS + DCS 16x16 icons only display on both shells in ASM programs - only DoorsCS supports icons on BASIC programs.

Icons aren't made for MOS, so I'm not sure why you even went down this road

Well MirageOS supports a couple of different headers, some with 15x15 icon 'buttons'.

OK sorry I had misread - correct I don't believe that MOS support BASIC icons, only DCS does. Both can be run by either shell with the Hybrid Header: https://dcs.cemetech.net/index.php?title=BASIC_Header#.22Hybrid.22_MOS-DCS_Header

In order to use icons in the first place in MOS you need the Graphical Buttons GUI too: https://www.ticalc.org/archives/files/fileinfo/139/13971.html