There are two reasons for Avalugg to run Refrigerate, so in looking at the viability of the theorymon we need to decide if those reasons are good reasons.
Unblockable Rapid Spin is the first thing everyone brings up, but is that really that useful? As I mentioned earlier, Excadrill doesn't really have too many problems being blocked as is. Neither of OU's premier Ghost types, Aegislash and Gengar, can take a STAB Earthquake, which is always a strict possibility from Excadrill. Since switching them in is oftentimes a crapshoot, they don't actually block Excadrill very well. Defog is also a huge issue here; while answers like Bisharp exist, it's still a very reliable move that would give Avalugg huge competition.
I don't see this being a good reason to use Avalugg.
Thus, we look at the second reason; Avalugg could make a good bulky attacker with Refrigerate Return and immense physical bulk. Return
does hit like a truck, and with that physical bulk Avalugg can force out some unboosted attackers. However, it's use seems to end there. Here's a few things to keep in mind;
- Avalugg cannot switch in on special attackers without heavy SpD investment, and even then stronger/boosted ones will still often 2HKO.
- Even with max investment in Def and HP, Avalugg still cannot reliably switch into many physical attackers, as a lot of them utilize Fire, Rock, Fighting or Steel moves anyway. Those that don't (or don't get STAB on them,) may boost on your way in anyway, or 2HKO otherwise.
- SR weakness (and Spikes/Toxic Spikes vulnerability,) really hurt Avalugg's bulk.
- If you somehow get Avalugg in safely, he can't really do much oftentimes anyway. He's obviously not a revenge killer, so bringing him in after something else on your team falls will probably place him in a position where the opponent can OHKO or cripple him anyway. If, say, you VoltTurn/Memento him in, he can probably fire off one Return/Rapid Spin before being forced out. Wonderful.
- Avalugg can't have every stat it needs at once. Despite gargantuan physical bulk, it needs investment if you want to live through the myriad super-effective physical moves being launched around OU on a regular basis. If you don't want to be 2HKO'd by, say, unboosted Manaphy's Surf, you need SpD and HP investment (and probably an Assault Vest.) If you want to make the most of Refrigerate Return and actually be threatening offensively, you need to use those EV's in Attack instead. There's just no combination of EV's that ensures Avalugg has the chance to be meaningful in any game.
These are the reasons I believe Refrigerate Avalugg would be too lackluster to work. There are too many things holding it back.