Update: smPlugin - Menu Addon
Due to what seemed like an increasingly popular demand I've thrown in the towel and finally written an 'addon' for smPlugin that provides an interface for your game so that players can save games 'on the fly' - much like they would usually be able to in visual novels and the like.
With thanks to @Malebranche for the idea and design:
It's very easy to use, simply download and include the smPluginMenuAddon.js file in your index.html (BENEATH smPlugin.js - you do need both files!), run your game and after 3 seconds or so the buttons and slot selection should appear and function as intended.
Note that you do still need to use *sm_init on the first page of your game to initialize your slots and game id - that is extremely important!
Other than that the addon should do the rest for you, though you can of course still use the smPlugin.js core commands, if you want to make an autosave or the like (just make sure you tell your players which slots you'll be overwriting!).
If at any point you want to be cruel and disable saving you can also use the extra command:
And of course, to re-enable saving:
This addon (in my eyes) makes save.js all but redundant, so unless there are any complaints I'll be considering discontinuing support for that rendition of the mod.