how much current do you need at 12v... I assume its more than a few ma?
a simple capacitor charge pump could double your voltage, but only supplies very small current
a lot of the simple single chip boost converters out there have a limit of 6 or 6.5 volts, so your SLA when fully charged, will exceed the design specification ... although perhaps you could use a series diode or two, to drop some of the excess voltage, keeping the voltage within the working range.
visit the Texas Inst website and get some free samples (they claim world wide) of the TPS61040 ... it's very simple to work with and can easily be used to supply up to 400mA at 12 volts from a 2 to 6v supply