A BandHelper user just pointed me to
1Chart, an iOS app focused on creating charts using the Nashville Numbering System. You can write basic NNS or Roman numeral charts directly in BandHelper, but 1Chart has a lot of built-in formatting options and special characters to show rhythm and other details.
1Chart is designed to integrate nicely with other apps, so after building a chart there, you can export it as a PDF into BandHelper, then attach the PDF to a song in BandHelper. Once you've done that, the PDF will sync to all the devices in your band automatically.
Like with all attached documents, if you want to edit the document later, you'd need to go back to 1Chart to make your edits, then export to BandHelper again, then attach it to the song again. (If you skip the last step, the updated file will appear on your device, but won't sync to the other devices in your band.)