[WJP] For mysterious reasons format will print an integer in decimal, hex, or octal, but not binary. Here's a procedure that does.
It works for negative integers as well as positive.
If the argument is not an integer it throws an exception.
It will use a sufficient number of bits regardless of the size of the integer.
It takes an optional second argument specifying the
number of bits to use. If the required number of bits
is larger, this value is ignored. If the specified
number of bits is larger than the necessary number of
bits, the result is zero-padded to the specified number of bits.

 proc binfmt {k {cnt 8}} {
    if {![string is integer $k]} {error "argument is not an integer"}
    #Compute minimum number of bits necessary
    #This is the integer ceiling of the log of the number to the base 2.
    set bits [expr int(ceil((log(abs($k)))/(log(2))))]
    if {$bits > $cnt} {set cnt $bits}
    #Now compute binary representation
    for {set i [expr $cnt -1]} {$i > -1} {incr i -1} {
        append s [expr ($k >> $i) & 1]
    }
    return $s
 }

 proc demo {} {
    puts "binfmt 3         = [binfmt 3]"
    puts "binfmt -3        = [binfmt -3]"
    puts "binfmt 3 0       = [binfmt 3 0]"
    puts "binfmt 3 4       = [binfmt 3 4]"
    puts "binfmt 25        = [binfmt 25]"
    puts "binfmt 1025      = [binfmt 1025]"
    puts "binfmt 69125     = [binfmt 69125]"
    puts "binfmt 69125 24  = [binfmt 69125 24]"
 }

 demo

Here is the output of the demo script:

binfmt 3         = 00000011
binfmt -3        = 11111101
binfmt 3 0       = 11
binfmt 3 4       = 0011
binfmt 25        = 00011001
binfmt 1025      = 10000000001
binfmt 69125     = 10000111000000101
binfmt 69125 24  = 000000010000111000000101