There are several, though not all of them are harpsichord treatises and the topic is rather wide-ranging. One thing to look to is Hotteterre’s Principes and compare this with Saint-Lambert. I shall tell you why. Both indicate that, in conjunct motion, pairing notes was a natural thing in all walks of musical life. Hotteterre uses phonetics to demonstrate: tu-ru, tu-ru, tu-ru, pairing across the beat; Saint Lambert does it by placing fingers 3 & 4 (rh ascending) on the same lateral plain, indicating the pairing. Jean-Pierre Freillon-Poncein (1700) is also interesting:
In all types of bars, to make the melody more agreeable to the ear, when there are only four crotchets or four quavers, they should be played tu tu ru tu and ru on the following note; when the tempo is very fast, it is necessary to play them tu ru tu ru, with tu on the notes that follow them.
Both Hotteterre and Freillon-Poncein’s treatises imply a phrasing across beat and bar line. In terms of keyboard music, such an approach would be best achieved through fingering. Thus, in ascending passages, rh: 1-2, 3-4, 3-4, 3-4; lh: 4-3, 2-1, etc.; in descending passages: rh: 5-4, 3-2, 3-2, etc.; lh: 1-2, 3-4, 3-4, etc.
Slurs are different since they can indicate a variety of techniques. In some instances, they are not slurs but liaisons and indicate a slight overlap between the two notes. We see this in particular in connexion with the tierce coulé and port de voix, though Marchand, Clérambault et al. also use the effect.
Otherwise, it might indicate even notes when notes inégales would be normal or, as you say, it could mean a reversed inégalité, whereby a Lombardy rhythm is played.
The problem is that no one knows for certain and the approach is down to that arbiter of taste, le bon goût. I suspect it also has something to do with the nature of the music. I would be wary of using using a Lombardy rhythm in som fast music since it would be inelegant, but in courantes and sarabandes, it can be quite an arresting effect.
There’s a lot of information out there and all the treatises I mentioned can be found at Gallica, which is run by the Bibliothèque nationale de France.