>>I wasn't about to do sth: Hier ist "sth." ein Platzhalter für etwas ganz bestimmtes.
>>I wasn't about to do anything. -- Ich dachte nicht daran, überhaupt irgendetwas zu tun.
The 'do sth.' issue arises only because of the LEO format. Compare other phrasal verbs such as 'be about to' (regular, nonemphatic), 'be going to,' 'be ready to,' 'be prepared to,' 'be willing to,' 'be inclined to,' etc.; all these should probably be handled in the same way.
In all those cases, the placeholder for the following verb could probably just be left out. But the catch may be that, on the English side at least, 'do sth.' vs. 'doing sth.' may be a way of telling the user whether it needs to be an infinitive or a gerund. (Compare 'be used to': We're used to eatING fresh vegetables every summer, vs. 'used to': We used to eat vegetables from our grandmother's garden when we were kids.)
It probably would have been clearer all around if LEO had from its inception included abbreviations such as [+ dat.] [+ acc.], [+ infin.] [+ ger.], [trans.] [intrans.] [reflex.], etc., instead of just relying on the less specific 'sth./so./one/one's' system, which hasn't always been used consistently over the years. But changing now would probably be a fairly major undertaking. In the medium term, however, it could be useful to draw up some internal guidelines for how to denote grammatical categories, if that hasn't already been done.