How to use BASH variables with "local", but less bloated?

If you're going to use local to declare each variable separately I'd be inclined to coalesce the first two lines (much like the POSIX variant of export PATH; PATH=/bin/…):

local foo; foo="$(bar)"

I'm not so sure that the third line's error case will ever get executed; if bar failed, then the assignment foo="$(bar)" will exit, and if it succeeds then it will never need to be executed. On this basis it seems better to tag the variable with readonly:

local foo; foo="$(bar)"; readonly foo

Unfortunately this isn't much different to your original code. I've put mine on a single line; yours is on three.

If you don't mind subverting bash syntax you can create a function to perform the assignment and set the readonly attribute. Here's a worked example.

First create a function called assign that assigns a variable to the output from a command and sets it to be readonly. An assignment error will send a TERM signal to the program itself, causing it to abort.

assign() {
    local _n="$1" _ss
    shift

    local _errTrap="$(trap -p ERR)" _setE=$([ -o errexit ] && echo 'set -e')
    trap '
        echo "TRAP/ERR: PID=$$ bad assignment to variable: ${_n:-<unknown>}" >&2
        kill "$$"
    ' ERR
    set +e

    eval "$_n"'=$("$@")'
    _ss=$?

    readonly "${_n}"

    if [ -n "$_errTrap" ]; then eval "$_errTrap"; else trap - ERR; fi
    if [ -n "$_setE" ]; then set -e; fi

    return "$_ss"
}

Now let's use it:

echo 'Assign x=$(date)'
assign x date
echo "x=$x"

And:

echo 'Assign y=$(false)'
assign y false
echo "y=$y"

And finally:

echo 'Fail to assign to readonly x'
x=123

You can use this approach in a function by localising a variable and then assigning it:

f() {
    local d; assign d date
    echo "f(): d=$d" >&2
    …
}