What kind of potting soil did you use? Was there a chance it dried out during that funky weather?

I also live in the Seattle area and grow ALL my maples in containers. Several of them DID experience some drying and dying of foliage in the repeated heat waves simply because I neglected to water quickly enough!! We Puget Sounders are not accustomed to needing to water as early as April, which is typically a pretty wet month for us (and with much cooler temperatures)!

Usually, a JM that has experienced some slight drought will releaf pretty rapidly. All of mine are doing great now and most have produce a second flush of foliage. But if the new guy was not yet established and dried out too much, it may have been more drought than it could tolerate. If it has survived you should see some swelling of new buds long the stems and even the start of new growth. Can you post any photos? Especially close ups of the stem/branch tips?

I'm not sure I'd hold out a lot of hope........sorry:-(( That looks pretty thoroughly dried out.

FWIW, I would NOT recommend planting a JM and a juniper in the same container. Providing enough water to keep the maple happy will result in an over-watered juniper!! Just not good companion plants.