Guidance at Magic Carpet Travel seems quite clear. Regarding sponsorship in general:
The visa process is in two distinct parts. First you have to obtain the visa permit, which can only be issued by the Foreign Ministry (FM) in Iran. You cannot lodge an application with the Iranian FM directly, nor can you currently lodge an application through an Iranian Consulate. You therefore have two ways of obtaining an Iranian visa permit:
You can ask your Iranian friends or relatives to apply for a visa on your behalf in Iran, by lodging an application in person, at the Foreign Ministry in Tehran. They will need to undertake the responsibility of acting as your host for the duration of your stay in Iran.
Alternatively, you can use an Iranian Visa Service to take care of all the details and get your visa for you. Iranian Visa Services can often get you a visa to travel at short notice.
But in answer to Do I have to have my itinerary organised before I apply for a visa?:
No, not unless you are a UK, Canadian or US citizen. Visas for above mentioned passport holders are subject to particular regulations. At present the Iranian Foreign Ministry will only issue visas for the above citizens to travel to Iran as part of a pre-booked, guided tour.
By guided tour we mean that you must travel either as part of a bigger tour group, or on a tailor-made individual tour designed to your own requirements. You must submit your itinerary in advance and adhere to it exactly.
So as far as independently goes, the answer is no.