[AgentZero]

It'd be especially handy for multiplayer where you sometimes end up building lots of ships to trade, but don't want them to have the highest tech, or even if you're just building scouts that you know have a good chance of winding up captured, you don't want to give your enemies free tech.

The thing is, the way the tech works in SE5, each tech level doesn't have it's own entry, there's just the one entry for a certain field (ie: Meson Blasters) and then a formula describes what happens as you increase in tech.
In SE4, you had seperate entries for each tech level, so each level of tech could be seen as a seperate object, so in SE4, you had 10 different Meson Blasters, each with it's own stats, whereas in SE5 you have 1 Meson Blaster with stats that vary according to your tech level.

So, while it'd sure be nice to use older tech, it's probably not feasible in SE5.

Edit: Unless there was an option in the design screen that said 'Use Current Tech' or 'Use Tech X Generations Old' where X is a user definable number. Then a formula could be applied to the current techs to 'reverse' them to the level you desired. Of course, it'd be kinda clunky if you wanted to use engines that were 5 techs old, APBs that were 10, shields that were 7 etc., but it's the only way I can think of doing it.