OP's query:

We could've met tomorrow. Maybe we'll hang out when you get back.
Is the use of could've fine here?

The use of could've is fine in: We could've met tomorrow. This sentence is a condensed third conditional containing only the main clause.

Let's look at the structure of the second and third conditionals Ref. *learnenglish.britishcouncil.org

Second Conditional structure is usually:
if + past simple >> + would + infinitive. In third conditional sentences, the structure is usually:
If + past perfect >> would have + past participle.

OP's sentence can be rewritten in the following manner:
'1. If you weren't traveling, we could've met tomorrow.

In this sentence the if clause is a hypothetical or an impossible clause (because the friend has already decided to travel). So it is a mixed conditional. "If clause" is Second Conditional and main clause is third conditional.

Now if we rewrite the sentence in this manner:
'2. If you had not decided to travel, we could've met tomorrow..
Now the if clause is in Past Perfect - the friend had already decided to travel and the decision cannot be changed.
The third conditional often expresses regret about past actions.

OP's query:

We could've met tomorrow. Maybe we'll hang out when you get back.
Is the use of could've fine here?

The use of could've is fine in: We could've met tomorrow. This sentence is a condensed third conditional containing only the main clause.

Let's look at the structure of the second and third conditionals.
Ref. learnenglish.britishcouncil.org

Second Conditional structure is usually:
if + past simple >> + would + infinitive..
In Third Conditional sentences, the structure is usually:
If + past perfect >> would have + past participle.

OP's sentence can be rewritten in the following manner:
'1. If you weren't traveling, we could've met tomorrow.

In this sentence the if clause is a hypothetical or an impossible clause (because the friend has already decided to travel). So it is a mixed conditional. "If clause" is Second Conditional and main clause is third conditional.

Now if we rewrite the sentence in this manner:
'2. If you had not decided to travel, we could've met tomorrow..
Now the if clause is in Past Perfect - the friend had already decided to travel and the decision cannot be changed.
The third conditional often expresses regret about past actions.