Indeed, it's very tough to find reference for you on the Internet as the expression is very outdated. I conducted google search but no results found. So I can only judge based on the logics of the financial industry.
Previously, I thought the term is seconder or resolutioner, but now I changed my mind, what the author wanted to express by using 赞成人 is 赞助者、资助者 who are like angel investors. They provided financial support to the company and the company places shares to them as reward in return.
We can see the order of priority from the source text: 发起人->赞成人->酬应->公积->办事人员. 赞成人 is the second most important role. So I'm sure it will absolutely be ok to use "patrons" as the meaning of seconders, supporters, proponents and resolutioners are all included in this word.

今天议案是支配公募股，因为股份总额是五万股，发起人每人得五百股，一共五十个人，照算已去了两万五千股，还有四十个赞成人，每人得二百股，一共八千股，那么还有一万七千股，除了酬应股五千股，公积股五千股，办事人员酬劳股三千股，只余四千股了。这四千股就是公募股.
The agenda for today is publicly offered shares allotment.
The total number of shares is 50,000:
500 shares per initiator x 50 initiators in total = 25,000 shares;
200 shares per resolutioner/seconder x 40 patrons in total = 8,000 shares;
50,000 shares – 25,000 shares – 8,000 shares = 17,000 shares remaining, and
17,000 shares – (5,000 remuneration shares, 5,000 reserve shares, 3,000 compensatory shares for clerks) = 4,000 shares remaining. And the remaining 4,000 shares will be publicly offered.
[Note: do not translate 发起人 into sponsor as in the financial sector sponsor refers to 保荐人]