Hi Carsten and Toomas,
the 
set pcplusmp:apic_panic_on_nmi = 1

is already present in /etc/system.d/_omnios:system:defaults and I've added there  a

set snooping=1 

which should enable the deadman timer.

If this is not enough I'll re-enable ILO (iDrac on HPE systems) and try with the ipmitool  as per your suggestion.

Thanks to both, I'll keep you posted.

Maurilio.