Seems to work OK. I just MEMCOPYed 4 consecutive V-registers to a UDT instance consisting of two signed DWORDS.
Only caveat being that the MEMCOPY insisted I specify the data size in destination elements (1 in this case).
STR C77
MEMCOPY V0 TestUDTInstance0 0x30000 1
#BEGIN UDT_CONFIG
TestUDT
// Field Name, Data Type, Memory Layout, Read/Write, Display Format, Data View Detail
dw0, SDWORD, 0:0, Read-Write, Native, Short
dw1, SDWORD, 1:0, Read-Write, Native, Short
#END
#BEGIN MEM_CONFIG
TestUDTInstance TestUDT decimal 16
#END