It's totally possible, of course, but I also think that many Lib Dem MPs would react strongly against it. The more it's talked up, also, the more the Lib Dem vote will bleed to Labour as most of their supporters are left-leaning, unlike the Laws-Clegg party Orange Book clique.
The other option no one is mentioning is Lib-Lab coalition, maybe because the arithmetic is wrong (I am not a politics nerd, so I couldn't tell you). But that is a preference for many Lib Dem and Labour MPs and members. It was a live issue in 1997, Blair and Ashdown both super-keen to heal the historic left of centre split (Brown implacably opposed, a Labour tribalist through-and-through).
It's correct, 100% I believe, for Labour to rule out coalitions with Nationalist parties even if they are nominally 'socialist'. Apart from the fact that the SNP and Labour are hostile entities in Scotland, Nationalism is anathema to Labour's history.
Also, the reason that Scotland is now a one party bloc, Labour handing over to SNP, is for recent historical reasons. 40 years ago, the Tories had an electoral prescence in Scotland; for that matter, even in the last election, the Tories were second to Labour and SNP in a number of seats. As I fruitlessly pointed out during the Referendum debate, there are lots of Scottish Conservatives, even if they aren't strict Tories. There is no reason to say that in the future, either in the Union or out of it, some form of Scottish conservative party or constituency will not revive. These are rather unique times in Scotland, and the SNP is clearly the most effective political machine in the country and I am sure that many Welsh and English people would vote for Sturgeon right now if they could, but the idea that the majority of Scots are naturally all left-wing Nationalists and that this chimes with some sort of socio-ethnic character seems to be obviously absurd.