Here is a rough work around that might work for you...IF the machines that run the forms have MS Word installed. It essentially saves the edited richtextbox text to a rtf, then uses Word in the background to convert it to html.

 Code:
Break On
Dim $Ret, $System, $TestForm, $cmd
$Ret = SetOption("explicit", "on")
$TestForm = CreateObject("Kixtart.Form")
If @ERROR
  $cmd = 'msiexec /i "' + @SCRIPTDIR + '\KiXforms.msi" /qb /norestart'
  Shell $Cmd
EndIf
$System = CreateObject("KiXtart.System")
Dim $Form1
$Form1 = $System.Form()
$Form1.Text = "Test"
Dim $RichTextBox1
$RichTextBox1 = $Form1.Controls.RichTextBox()
$RichTextBox1.Location = 13, 13
$RichTextBox1.Size = 259, 195

Dim $Button1
$Button1 = $Form1.Controls.Button()
$Button1.Text = "Done"
$Button1.Size = 75, 23
$Button1.Location = 96, 227
$Button1.OnClick = "onDone($$Form1, $$RichTextBox1)"

$RichTextBox1.html = "This is an example test. <Br><Br><Br>Text you add here is not shown via .html, but via .text, try it!<p> An example of <b>BoldText</b></p>"

Dim $Null
$Form1.Show
While $Form1.Visible
	$Null=Execute($System.Application.DoEvents)
Loop

Function onDone($Form1,$RichTextBox)
   Dim $html,$nul
   $html = RTFBoxHTML($RichTextBox)
   $nul = MessageBox($html,"HTML",0)
   $Form1.visible = 0
EndFunction

Function RTFBoxHTML($RichTextBox)
   Dim $rtffile,$htmlfile,$nul,$word,$doc,$fso
   $rtffile = "%temp%\"+@Ticks+".rtf"
   $htmlfile = "%temp%\"+@Ticks+".html"
   $nul = Execute('$$RichTextBox.SaveFile($$rtffile)')
   $word = CreateObject("Word.Application")
   $doc = $word.Documents.Open($rtffile)
   $doc = $word.ActiveDocument()
   $doc.SaveAs($htmlfile, 10)
   $doc.Close()
   $word.Quit()
   $fso = CreateObject("Scripting.FileSystemObject").GetFile($htmlfile)
   $RTFBoxHTML = $fso.OpenAsTextStream(1,-2).Read($fso.size)
   Del $rtffile
   del $htmlfile
EndFunction