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.