@SuiteBender: Hi, If you want to keep using existing way simply add the inlinehtml or Richtext field to the suitelet form in your suitelet code(this is kind of server side) -- See form.addField(name, type, label, sourceOrRadio, tab) --(Check nlobjForm and nlobjField in Help documentation.)
Then do as @rustyshackles said, update that field from your client script code.
Btw, can't you capture the data from budget records in your suitelet code itself ? Collect the data in suitelet code & set the html field default value to your html( converted to html string)