Would an isometric deadlift be a good substitute for a normal, barbell deadlift?
Isometric exercises do have their place in training, but can never replace the lift itself.
The problem with isometric exercises is, that you don't have any range of motion, you just hold the weight there and contract your muscles. While that surely has a positive effect on the muscles in that position, it doesn't help much in other parts of the lift.
For example, if you're doing an isometric deadlift at knee-height, that won't really help you at lockout. Neither will it improve your ability to get the weight of the ground much. It would provide some benefits though, as you'd strengthen your core and probably get better at the knee-high part of the deadlift.
So, basically no, isometric exercises are not good at replacing a lift, but they are a tool to improve certain parts of a lift. If you can identify weak points in your deadlift, you might still improve it that way, even without a barbell.
The shorter answer is NO.
Dumbbells, by design, exercise only an arm. Barbells, by design, exercise both arms. Since deadlift is a 2-arm exercise, it cannot be completely and effectively done by dumbbells.
This doesn't indicate that you cannot use dumbbells; you can use the same dumbbells for various exercises. And you can even use it for deadlift-like exercises; you just cannot call those exercises deadlifts and you certainly will not obtain the benefits of deadlifting.
Use whatever you have around you, till when you have access to your beloved barbells :).
Whatever you do, don't stop exercising.
Now, go lift some weights :).