Rapiers and sideswords seem to have been commonly paired with main gauche daggers but did this "fighting style" exist before them? I could imagine an arming sword or messer working quite well with a dagger but I've never seen or heard of them being used with one.

With rondel or bollock dagger, parring would be harder than with main gauche because of the lack of hand protection.

Albrecht Dürer shows Messer and dagger in his Οπλοδιδασκαλια (Albertina MS 26-232) - quite nifty techniques actually (see Dornhoffer's study) :

http://wiktenauer.com/images/8/85/MS_26-232_86r.png

Seeing how close Dürer is from previous sources (such as those now gathered in the so-called "Codex Wallerstein"), such combination of weapons is quite likely pre-1500.

In a similar vein I've found 14th c. artwork that shows a sword being used with a cloak wrapped around the hand. Heck, now that I think of it I vaguely recall a Viking era reference to somebody using a hand axe as a parrying weapon.

From a practical point of view sword and dagger is really good for dueling but in an environment where I might have to watch out for thrown weapons I'd probably opt for a buckler or shield.

AFAIK there's not a lot of explicit instruction on sword and dagger in the currently known corpus but they were definitely familiar with the idea of wielding daggers and swords at the same time... in some cases they weren't just dual wielding either, they were triple wielding!

http://wiktenauer.com/wiki/File:Cod.I.6.2%C2%BA.1_8v.png

http://commons.wikimedia.org/wiki/File:Ms.Thott.290.2º_078r.jpg

http://commons.wikimedia.org/wiki/File:Ms.Thott.290.2º_119r.jpg

Yeah, once you know the basics of the system a lot of wild things are possible with KDF.

Here's an image of 14th c. sword and cloak,

http://manuscriptminiatures.com/4750/11306/

Now I know it's not a fechtbuch but a lot of technique is recognizable in artwork, in this case the swordsman is standing in luginslant/tag, very similar to the fellow on the top/right of this image from Talhoffer,

http://daten.digitale-sammlungen.de/0002/bsb0...;seite=244

For arming sword and dagger I don't actually use much single dagger technique, instead I borrow heavily from sword technique. The dagger's all strong and about 1/3 the length of the sword so parrying works about the same even if you have to be a bit more precise, other than that I'd mostly be using the dagger against a full length sword which adjusts the timings and rhythms in the right way for it to work.