My understanding was that you where doing one HTML for multiple Leads, but from your previous comment is seems like it's one HTML per lead, right?
Do you have a large volume of leads/HTML combinations to deal with?
If so, then, the Request Campaign that Murtza Suggested might be your best option. My understanding is that you have only one Token value passed by API Call - so It would require 1 API call per lead.
Another alternative could be tu store the HTML Data within the lead record. Then you can 1) update Lead with HTML using API 2) Request or schedule campaigns in batch. In this case (using lead field) I think you would be limited in the lenght of 3,200 characters and I'm not 100% sure how encoding-decoding could affect the structure.
Cheers