For what it's worth, I'm in Melbourne and I have a Bloodgood in full sun with little wind protection and its leaves are not crispy at all. The key is daily watering when it is warm and dry. Most Japanese maples can handle the heat with enough water.
Granted that JM can be grown in Melbourne with adequate water - most of the time but there are a lot more variables that you appear to be allowing for.
Trees that have been in pots for a few years become more difficult to keep the water up to as spaces in the potting soil that used to hold water are now filled with roots.
Different sized pots will impact how much water the trees have access to. Obviously very small pots relative to the size of the tree may not hold enough water to last through a hot day.
Different potting mixes drain more or less and hold more or less water than others. A tree in the same sized pot may cope well in a moisture retaining mix but dry out in a more open potting mix.
Even in a single city like Melbourne there can be significant differences in micro climate. Bayside suburbs have higher humidity and lower temperatures moderated by the nearby ocean. Elevated areas are generally cooler than open, exposed areas. North- Western suburbs will generally be hotter, drier and windier. Leafy Eastern suburbs are cooler and more sheltered than newer exposed areas. Higher apartment terraces are more exposed to dry winds than ground level courtyards so there will be significant differences in how the same tree will cope in different parts of the metro area.
Watering habits can vary. Inadequate watering may allow the pot to get a little drier each day. When soil gets dry it becomes hydrophobic and difficult to wet properly. It may appear well watered on the surface but most of the water applied has run right through without properly wetting the roots.
As the temp rises and humidity decreases, here seems to be a point where Japanese maples just can't take in enough water to cool themselves, even though the soil is still wet.
Going back to the original question: From the pics I'd say these trees have excellent chance of recovery provided they receive proper care soon. Always soak a dehydrated pot in water for an hour, even overnight to ensure the tree gets water and the soil is properly wet. If the pots are too big to fit into a container of water the pots need to be well watered 3-5 times in an hour or so. The first few times will only wet the surface, even if water runs out the bottom of the pot. Subsequent water soaks deeper and deeper into the root zone. It is important that the entire pot be wet so keep watering intermittently.
FWIW, I water most of my pots morning and evening through summer and still get occasional dehydrated trees.
There are also some measures you can take to mitigate dehydration if it becomes a problem.
