According to 10.6.1 of the UEB code, the 'be' contraction can be used when the letters it represents form the first syllable of a word. According to 10.6.2, 'be' can be used when it is at the beginning of a word and followed by a letter, contraction, modified letter or ligature. The letters 'Be' form the first syllable of Beowulf, begin the word and are followed by a letter. So yes, you can use 'be' in 'Beowulf". If pronunciation is your concern, in 10.6.2, the 'be' contraction is used in the word Belinda which has the same beginning sound (at least to me) of Beowulf.