Nemeth Code does address font attributes. The only ones specifically defined are sanserif, regular, script, bold, and italic. In essence though, we are applying only bold and/or italic in our transcriptions. The rule says the font attribute is only shown if it has mathematical significance, although we do use bold and italic in narrative context also.
To indicate the highlighted digits with a long number you can do it either one of the two ways you suggested (I'm pretending like I didn't see your suggestion "c"). However, if you italicize the number, the italics is terminated by the use of another numeric indicator. 12345. [braille]#12.#3#45 Section 32