You could build a custom form that pulls in the Templates, add a choice of Rich Text or Plain and swap a MultiLine Text box for a HTML Editor. Then have two Send SMTP Email components, one set to use Html and the other Text in the Format dropdown,
Check the [ServiceDeskIncidentTemplate] Table if you want to use custom SQL
There is also 'List Incident Templates' (Symantec.ServiceDesk.Im.Core.Components.ListIncidentTemplateComponent) just pull in the Im.Core Library (Symantec.ServiceDesk.Im.Core.dll) and others for searching.
You could then add this as a Process Type Action to your Incident Page
Components
Send Email Component Via SMTP
LogicBase.Components.Email.SendEmailComponentViaSMTP
http://www.symantec.com/connect/articles/send-email-component-smtp
List Incident Templates
Symantec.ServiceDesk.Im.Core.Components.ListIncidentTemplateComponent