Turnover of tech volunteers is not necessarily a thing to worry about. There are lots of unemployed tech folks out there right now and you're better off leveraging their skills with a shorter-term project that trying to recruit them to a more permanent position. I find that many of these volunteers don't know what the future holds for them employment-wise and they are much more comfortable committing for shorter, finite periods of time.
Another important thing to remember is that documentation of protocols and procedures that comprise your job is not only useful for volunteer managment, but the organization as a whole. What happens if you suddenly cannot be at work for a prolonged or indefinite amount of time? What happens in the event of a natural disaster? I am an adamant proponent of not retaining information in people's brains, but getting it down on paper as an insurance policy of sorts against unforeseen circumstances. The added bonus is that it makes volunteer recruitment and management world's easier. It also makes it possible for me to go on vacation without co-workers hyperventilating the entire I'm gone.
And last but not least, I'd recommend you reconfigure/rethink your volunteer strategies. Given the skills you're looking for, do they all need to come in one person? Think about the economic climate and how that might impact your recruitment efforts. Leverage the current economic climate to your own benefit and define more discrete projects for volunteers. I've even heard of someone recruiting an IT Help Desk position with a volunteer who worked a corporate job and came in over his lunch break to help folks. Flexibility on your part is key. And the money saved by the organization in using volunteers is significant, so track it to justify your time spent if need be. You can also (depending on the project) have volunteers document the value of their time and number of hours to possibly use as a match for grants. The possibilities are endless. And ironically enough, it actually takes less time to manage short-term folks compared to a more all-encompassing position. Especially if you have your documentation in place and recruit the person with the most appropriate skill set. Also, get creative with who you recruit - what about college students who need to do internships for a computer science degree? What about nonprofits that train folks to start working in tech fields, like Relia Tech? What about corporations that give their employees time off to volunteer, like Wells Fargo and PG&E?
The following are some absolutely fabulous resources I've found on tech volunteers and which helped me to rethink how I was and could recruit volunteers. Best of luck to you!
Working With Technical Volunteers (http://www.techsoup.org/binaries/files/TechVolMan2001v1.2.pdf)
Managing Tech and Online Volunteers (https://sites.google.com/a/idealist.org/resources-for-organizations-msw09/Home/managing-volunteers)
more great resources (http://www.coyotecommunications.com/)