I do not believe any current (or even past 5+ years computer) has a default speed <= 400 mhz.
Some motherboards may enable you to manually configure the speed to that level. You are probably most likely to find them as an individually purchased component, vs. something that comes included in a mainstream prebuilt computer. Even then, though, I'd worry that whatever APIs the hardware lock relies on may have changed so much it may still be inoperable. You'd definitely want to try before you buy.
It may make more sense to negotiate with the vendor for an updated license / lock, or if the addon vendor is no longer in business perhaps you can find on the internet a version for which the hardware lock has been disabled.