The T1500s come with a utility called 'makeflash' (which lives in /sbin and also comes on the CD). You can use this to generate a flash filesystem image from a tar.gz, which can either be local, or an NFS / SMB file. This utility generates all the necessary checksums too.
1. Flashing with a Slackware 8.0 image doesn't work, even with the calculated checksum. This suggests to me that there is a further check made on vmlinux itself.
2. Having dissected the leo.boot and vmlinux files supplied with the unit, I suspect that leo.boot checks vmlinux for the string "xxxxyyyyWYSEwysexxxx" which is duly included in the custom kernel they use.
3. Leo.boot is in fact a Win CE related thing, with a queer bolt-on to allow linux boots. The normal Win CE leo.boot is about half the size. It uses the Eboot protocol for both ethernet and parallel port flash programming. There are some free Linux tools available to format images correctly for replacing leo.boot and also the OS image, but they rely on you supplying a suitably compiled object version of the bootrom.
Be careful when running it though - it is possible to flash the thing to a state in which the only way to get it booting again is to use the Wyse-provided DOS util / laplink cable setup.
Also, if anyone manages to get makeflash running on something which isn't the terminal, could you let me know how? AFAICT, it's trying to link with a very old glibc and libforms.You should be able to flash-update the T1500 a few 100,000 times.
Apparently the T1500 is a Wyse WT3X15, based on what they call the 'Leo Board'. You can download a DOS reflashing util, and some docs for it from the UK Wyse website, in the Customer Support section's firmware page. It requires a parallel laplink cable.
After running the mknod commands below, you can then mount /dev/fnand and /dev/fnand1, which are both 16Mb flashfs partitions. You can also mount /dev/fram, and write to it, however the data disappeared after umounting & remounting. It may have use as swap space.
mknod /dev/fcard b 63 0 mknod /dev/fcard.fmt b 63 128 mknod /dev/fcard1 b 63 1 mknod /dev/fcard1.fmt b 63 129 mknod /dev/fnand b 62 0 mknod /dev/fnand.fmt b 62 128 mknod /dev/fnand1 b 62 2 mknod /dev/fnand1.fmt b 62 120 mknod /dev/fnor b 65 0 mknod /dev/fnor.2m b 65 128 mknod /dev/fram b 64 128