Yes, indirectly, it is possible. I can think of a couple of ways.
You cannot assign a hotkey directly to a equalizer profile, but you can do either of two other things to achieve the same result:
1. Assign a hotkey to a ZONE. Each Zone you create has DSP settings configured in it. So if you wanted to switch between 3 different EQ presets, you could create 3 zones, assign an EQ preset to each one, and assign a hotkey to each zone.
See this thread for an example of assigning hotkeys to zones:
https://yabb.jriver.com/interact/index.php?topic=120582.02. You can assign a hotkey to execute a command line program, and use that to execute a MCWS command. There is an MCWS command to select a DSP Preset: Playback/LoadDSPPreset. Save your entire DSP configuration, including the EQ settings, into a DSP preset, and then those presets can be selected with a hotkey without changing zones. Each hotkey will trigger a different command line string holding the correct MCWS command for that preset.
See these two links for more info on this technique:
https://yabb.jriver.com/interact/index.php?topic=110859.0https://wiki.jriver.com/index.php/The_Command_LineHowever, from your description, hotkeys seem unnecessary. If you use a dedicated equalizer setting for each output device, just create a zone for each output device (Speakers, Headphone1, Headphone2, Headphone3) assign all the settings you want in each zone, and just select the zone you want when you begin playback. This is the way most people do it. The settings persist in each zone independently, so you needn't go into DSP Studio each time. Just configure each zone once, and then right before you start playback just click the zone (device) you want to use. Simple.
The interface makes it easy to select zones both in standard mode and theater view. So to create these hotkeys, you're going to a lot of work to avoid one click.
I hope this helps...