Depending upon the size of the P/C agency, it's product mix, and market alignment - a life producer can find great opportunity working with a P/C shop. There are many variables to consider - however suffice it to say that, in general, if you take overall agency revenues and multiply by .02 you will come up with a reasonable estimate of the value that can be derived by cross selling life insurance through that book.
Performing a great deal of due diligence up front, and from that diligence deriving an estimated 'revenue value' for life sales is important - because it will predicate what the relationship should look like, and, in turn, answer your question regarding maintaining 'independence' as you mentioned in your question.
For example, if the economic opportunity is small, say, less that $50k per year - this would naturally lend itself to scoping an arrangement that has a higher degree of independence for you. To the other extreme, say if the opportunity was closer to $300k per year or more, naturally an agency would be more inclined to 'institutionalize' the relationship and install conditions and parameters that would provide a lesser degree of independence for you.
To the issue of independence, you must be clear on how you define that as well. It means different things for different people.
I would encourage you to start with partnering with the agency in identifying the overall opportunity, develop goals and milestones for results, expectations around confidentiality, coordination of efforts within the same account, and, the often overlooked "exit strategy" which defines how both parties will exit the relationship if it doesn't work out - and do so in such a way where no one's 'rice bowl gets broken' and there is no chaos created that could impact a client.
Hope that helps.