So, I phoned the mutual fund company and they confirmed that they are not a bond fund, which from what I can see on the 'foreign' notes page of the self assessment guidance is the only thing that would bar the dividends from being declared in the 'dividends from foreign companies' box.
I also spoke to the HMRC technical adviser line today. I said 'I am receiving dividends from something called a mutual fund in the USA, do I report the dividends as dividends from foreign companies?'. The adviser said 'yes'. I then said 'I also receive capital gains distributions, are these treated like normal capital gains under the UK rules?'. Again, the adviser said yes and that as long as they were under the annual allowance limit that there was no need to report them.
Whether the adviser is actually right or not, I don't know, but I feel more comfortable now knowing that I've asked the question clearly and directly to HMRC and have recorded the date and time of the phone call.