I've given this some more thought. I believe, though am not certain, that income from an umbrella company would be counted as non-salaried employment. Since we would be applying with a combination of salaried and non-salaried employment, that means applying under Category B. By those rules:
Where the person is in non-salaried employment – the level of gross annual employment income relied upon in the application can be no greater than the annual equivalent of the person’s average gross monthly income from non-salaried employment in the 6 months prior to the date of application, regardless of whether that employment was held throughout that period. The calculation will include all non-salaried employment undertaken during that 6 month period, and will be calculated over that 6 month period regardless of how much of that period was spent in employment.
Let's say I get a day rate of £350 and work 20 days/month, which works out to £7,000 monthly income. (Minus some fee from the umbrella company.) If I understand correctly, on two months' income, my annual equivalent would be:
(0 + 0 + 0 + 0 + 7,000 + 7,000 ÷ 6) × 12
= (14,000 ÷ 6) × 12
= 2,333.33 × 12
= £28,000
Of course, I really,
really don't want to get this wrong!

I'd rather miss out on a good role than create trouble for our next FLR(M). If anyone can spot something I'm missing or could double-check my reasoning, I'd really appreciate it!