There was strong opposition in the early/NUR days of DOO implementation, but perhaps a national campaign wasn't prolonged or coordinated enough. There were a lot of issues in play around the time of the 1982 pay deal, in which they maybe lost sight of the long term significance and consequences of DOO on Bed Pan. Raising the low wages of many grades was a major consideration at the time though. You say they did 'very little', perhaps not enough. The book 'British Rail 1974-1997: From Integration to Privatisation' gives the impression that the NUR were firmly opposed to DOO, particularly in the case of Bed Pan.
I'd be interested to know what you're citing to say "
ASLEF did prevent [DOO] introduction on the Waterloo and Euston suburban services". Certainly in the case of the former the agreement against DOO was signed with the RMT, it's available online:
https://www.rmt.org.uk/news/publications/south-west-trains-doo/swt-doo.pdf