Hey Steve,
If you want to use requestCampaign API for the usercase you describe, then you'll have to issue a separate request for each lead that you want to email.
An alternative would be to use importToList, which allows you to specify a unique token value for each lead that you list in the request. Assuming the leads you want to email are already in MLM, you would perform importToList with importListMode=LISTONLY, then list your leads' email address along with the values for any associated tokens in importFileHeader. Set campaignName equal to the batch campaign that will be delivering the email and then perform scheduleCampaign to kick off this batch campaign.