Thanks to everyone who's looked at this and had a play around.
I have finally found a couple of hours to have a play around with it, just to proof of concept what I have been thinking about
See current change log batch:
Tue Dec 6 21:22:23 UTC 2016
To use the scripts you'd have copied the 'u-boot_tftp' into your /tftpboot/slackwarearm-current/ directory (a la the INSTALL_ docs), then load and execute them from u-boot as follows:
# Banana Pi
dh;dh;dh;tftp 0x52000000 slackwarearm-current/u-boot_tftp/bananapi
# Banana Pi Pro
dh;dh;dh;tftp 0x52000000 slackwarearm-current/u-boot_tftp/bananapipro
# Trimslice Pro:
dh;dh;dh;tftp 0x408000 slackwarearm-current/u-boot_tftp/trimslicepro ; source 0x408000
"dh" is short for "dhcp". The reason I have it thrice is because there's some oddity on my LAN switch where by it seems to be slow at picking up that something's connected to a port-- it happens for any device on any OS; so you probably don't need so many dhcp calls there.