> Can csound read its csd script from its stdin, to avoid the
> tmp file ?  I tried the obvious:
>   csound -
> but that didn't work...

There is a workaround

#!/bin/bash

csound -odac /dev/stdin yourscore.sco <<'EOF'
sr     =  44100
ksmps  =  64
nchnls =  1

instr 1
aout rand 5000
out aout
endin
EOF

> ; i 1 0 31557600  ; causes csound to hang immediately :-(

It works with a recent version of csound. In general, it is enough an
empty score:

touch empty.sco
csound -odac /dev/stdin empty.sco <<'EOF'
...
; trunon or alwayson opcode
...

There is also the INF macro:

  i1 0 $INF

If you can use the current git version, it is possible:

csound /dev/stdin << 'EOF'
<CsoundSynthesizer>
<CsOptions>
; -d -m229 -o dac -T -F midifile.mid
; -d -+rtmidi=alsa -m0 -M0 -iadc -odac
; listens on Midi-Through 1 = 14:1 ready for use in muz
-d -m0 -M1 -iadc -odac
</CsOptions>
<CsInstruments>
sr = 44100
ksmps = 128
nchnls = 2
0dbfs = 1
gicha2bank ftgen 0,0,16,2, 0,0,0,0, 0,0,0,0, 0,0,0,0, 0,0,0,0
...
EOF

Another possibility (in git) is

csound <(make_csdfile_script)

tito