VST3 actually has native support for converting a formatted value back to a normalized parameter value, so it should work automatically for any VST3, assuming the plugin supports it.
Additionally, the REAPER extensions to VST2
support this via effString2Parameter. Some 3rd party plugin developers support these extensions (Voxengo for example, I think). Somewhat embarrassingly, among REAPER's built-in plugins, only ReaEQ, ReaSurround, and ReaControlMIDI support the 2-way extension. We'll add support in ReaComp as well.