I'd say it depends on how big the distinction is.
Adding this dialect as a separate language is certainly possible, but also comes with a caveat: this dialect has to be maintained independently. Which means, as new strings appear, usually some 10 per month, they have to get translated for both dialects. Translations added for nl-NL don't get automatically transported to be-NL. Which might cause more work than maintaining, say, 20 distinctions between these two dialects in the local database.
If you're ready to maintain be-NL, let me know, I'll add it to the translations, then.