yay i believe i may have _finally_ tracked down the issue with the DDR3 RAM initialisation - not the actual underlying problem with the PCB layout (which is down to impedance and is probably going to need insertion of a bank of... probably like... 60-80 resistors... *sigh*...) but with initialising the RK3288 RAM controller at the much lower clock rate of 200mhz.
i _really_ should have checked the rkclk_configure_ddr function in u-boot-rockchip's clk_rk3288.c file first, but i have to say it's been great fun doing reverse-engineering again. i basically took the rkbin/rk32/ 32_LPDDR2_200MHz_LPDDR3_200MHz_DDR3_200MHz_20150318.bin file which rockchip supplied - and which is confirmed as successfully initialising the DDR3 controller (at 200mhz) on the EOMA68-RK3288 board - and have been rapidly working it back towards c-code.
... probably unnecessarily as it turns out, because *actually* what i quite likely only needed was to add "200mhz" as one of the supported switch-statements in rkclk_configure_ddr! ooops....
now, it has to be said that if i had not got as far through the reverse-engineering as i have, i would not have had the information that i needed in order to easily work out the PLL settings for the 200mhz clock-rate.
the reason is quite straightforward: the proprietary binary is SIGNIFICANTLY more comprehensive in its initialisation, parameterisation, safety-checking and follow-up testing of the memory. i can see _where_ the u-boot-rockchip sdram initialisation comes from: many of the functions are the same but the key strategic ones are significantly improved.
so, anyway, it's late evening, so i cannot go round to my host's house straight away and test this out, but i will do tomorrow first thing :)
l.
--- crowd-funded eco-conscious hardware: https://www.crowdsupply.com/eoma68