Are there operators out there who take SEL private pilots and train them to commercial? Do small operators like banner tow, agriculture, skydiving do that at all? Is it way too presumptuous to ask? To specify, I'm particularly interested in the US, especially California.
With the airlines that do offer such programs, the cost is offset by having a pool of new pilots who they can train to their liking from the start rather than having to retrain existing pilots. They also tend to be large operations for whom the cost of having some trainees on staff isn't a major factor.
For a small operator it's typically too expensive to carry the burden of your training cost as well as employing you while you can't fly commercially (which you won't be able to unless you've completed that training.
So unless they have an opening for a non-flying job you can do, with training provided as part of your payment for that job, they won't be able to employ you.
As most jobs tend to require training and certification that involve aircraft, such a job would almost certainly be something like a desk job, answering phones, cleaning, stuff like that, not something someone interested in a flying job is likely to greatly enjoy, increasing the risk to the company that the candidate is going to quit on them.
The only way that would work is to hire on in a non-flying job for a flying school that has an aircraft you can fly as a PPL and fly it around and get training in lieu of pay.