I propose:
Type: variant
Subtag: tarask
Description: Belarusian in Taraskievica orthography
Prefix: be
Comments: The subtag represents Belarusian orthography as
published in Branislau Taraskievic's "Bielaruskaja gramatyka dla
skol" (Vilnia: "Bielaruskaha kamitetu", 1929, 5th edition).
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Michael Everson * http://www.evertype.com