Language fallback
Markus Giger & Marián Sloboda (2008) say «The way interviews were (and still are) conducted impressed me the most. While an interviewer would speak Belarusian, most interviewees responded in Russian, no matter which social strata they represented. This ‘bilingual’ interviewing sounds awkward to an outsider, but locals are apparently used to it»; if so, it seems most would be perfect be-ru bilinguals.
Other sources found an 80 % intelligibility of Ukrainian for Belarusian (but not with a formal intelligibility study, apparently).
Can we make "uk, then ru" fallback? I'm notifying some more translators.
My rant about Ukrainian was sarcastic, sorry. You should request a comment from the communities (be and be-tarask) regarding the change. Ukrainian is not an option here, only Russian.
My educated guess is: 1) the overwhelming majority of readers, and quite a portion of contributors would find that comfortable, and be silent, and never say 'thank you'; 2) certain vocal minority, while also arguably being more comfortable, would pointedly demonstrate their discontent, possibly even fill your inbox with hate-mail. You choose :).
Looks like you're right. ;-) I also see that 91 % of traffic from Belarus goes to the Russian Wikipedia; only 1 % ends up visiting the Belarusian Wikipedia. https://stats.wikimedia.org/wikimedia/squids/SquidReportPageViewsPerCountryBreakdownHuge.htm On the other hand, only 20 % of the traffic to the Belarusian Wikipedia comes from Belarus! https://stats.wikimedia.org/wikimedia/squids/SquidReportPageViewsPerLanguageBreakdown.htm#Belarusian
So it seems extremely likely that users of the Belarusian interface are equally comfortable with Russian.