Incorporating the option for owners (or the public) to make tax deductible donations to your capital campaign is an excellent way to allow potential contributors a choice in how they participate. This will likely require your co-op to partner with another entity who can receive tax deductible donations on your behalf.
A local community foundation is a great place to start. Green Top Grocery was able to set up a fund through a local community foundation called the Illinois Prairie Community Foundation. The main purposes of this fund is to purchase shares in the co-op for members of the community who cannot afford them and to provide outreach programs such as nutritional education and teaching kitchen programming. Owners of Green Top Grocery and members of the public can make tax deductible donations to Green Top via this fund. Of the nearly $1,500,000 raised by Green Top Grocery $150,000 of donations were made through this fund.
If a community foundation (which will often restrict use of funds as in the case with Green Top) is not an option, an alternative would be to consider a partnership with The Cooperative Development Fund of CDS. Since 2009 The Cooperative Development Fund of CDS has developed a system to help qualifying projects receive support through donation of charitable gifts.