Can you sacrifice a commander that has been turned into a creature to avoid losing it to an exile effect?

In Commander, the 'commander replacement effect' lets you redirect your commander to the command zone whenever it would go to any zone other than the battlefield or the command zone — including the graveyard. This is defined in the Commander rules (rule 903.9). Crucially, this applies to any zone change, not just exile.

So if your commander has been turned into a creature (e.g., by a continuous effect from an equipment, aura, or card like Lifeline), you can sacrifice it in response to a spell or ability that would exile it. Sacrificing sends it to the graveyard (CR 701.17), but the replacement effect lets you redirect it to the command zone instead.

The key timing requirement is that you must have an opportunity to sacrifice it before the exile effect resolves. You can activate a sacrifice outlet (like Viscera Seer) in response to an Oblivion Ring entering or an opponent's Path to Exile targeting your commander-creature, sending it to the command zone safely.

Concrete example: Your opponent casts Swords to Plowshares targeting your Boros Equipment commander temporarily animated into a creature. Before the spell resolves, you activate Altar of Dementia, sacrificing your commander. It would go to the graveyard, but you apply the commander replacement effect, putting it in the command zone. The Swords to Plowshares then resolves but has no legal target and is countered by the rules (CR 608.2b).

Note: If the commander is not currently a creature (and has no other type allowing sacrifice), you cannot sacrifice it — you need a sacrifice outlet that works on the relevant permanent type. Also, some exile effects have additional conditions, but the core strategy of sacrificing to the command zone is valid under CR 701.17 and 903.9.