The old BAA Member Pages (before December 2020) did allow observation dates to be set in the future. Indeed, that may even have happened rather often: from memory, if people forgot to set the observation date, the default was set to the current time with a granularity of 15 minutes, while the upload time was set precisely. All of that was faithfully migrated across to the new database, even though the user interface no longer allows new observations to be in the future.
For some images, it’s very important to record the observation date – for example, planetary images, variable nebulae, etc. But as you say, there are other types of observation (e.g. blink comparisons) where it’s more complicated. For some deep sky images, where the photons have been collected over many nights, it’s very complicated indeed.
I think it would be reasonable to set the “observation date” to the date of the most recent observation, but there’s no “BAA convention” on this. What’s most important is to provide the details in the explanatory text. I’m sorry that your explanation got missed in the migration – there were a small number of comments that had migration issues, mostly due to character encoding issues.