So with emu.item I can do this:

Code
dofile("../../arcbd_load_vram.lua")

emu.item(manager:machine().devices[":sl4:arcbd:arcbd_tms"].items["0/m_Regs[0]"]):write(0,2)
emu.item(manager:machine().devices[":sl4:arcbd:arcbd_tms"].items["0/m_Regs[1]"]):write(0,0xc2)
emu.item(manager:machine().devices[":sl4:arcbd:arcbd_tms"].items["0/m_pattern"]):write(0,0)
emu.item(manager:machine().devices[":sl4:arcbd:arcbd_tms"].items["0/m_colour"]):write(0,0x2000)
emu.item(manager:machine().devices[":sl4:arcbd:arcbd_tms"].items["0/m_nametbl"]):write(0,0x3800)
emu.item(manager:machine().devices[":sl4:arcbd:arcbd_tms"].items["0/m_mode"]):write(0,2)

emu.item(manager:machine().devices[":sl4:arcbd:arcbd_tms"].items["0/m_spritepattern"]):write(0,0x1800)
emu.item(manager:machine().devices[":sl4:arcbd:arcbd_tms"].items["0/m_spriteattribute"]):write(0,0x1f80)



There's a couple of other properties of emu.item that are interesting too, like .size and .count that are interesting to examine.

Item "0/m_pattern" has a size of 2 and count 1 while item "0/m_Regs[0]" has size of 1 and count 1.

Item "1/00000000-0000ffff" has a size of 1 and count 65536, and I can read/write into the vram with it.

Very cool. This lua engine is awesome!


Code
for i,j in pairs(manager:machine().devices[":sl4:arcbd:arcbd_tms"].items) do print(i,j) end
print(emu.item(manager:machine().devices[":sl4:arcbd:arcbd_tms"].items["0/m_pattern"]).size)
print(emu.item(manager:machine().devices[":sl4:arcbd:arcbd_tms"].items["0/m_pattern"]).count)
print(emu.item(manager:machine().devices[":sl4:arcbd:arcbd_tms"].items["0/m_Regs[0]"]).size)
print(emu.item(manager:machine().devices[":sl4:arcbd:arcbd_tms"].items["0/m_Regs[0]"]).count)
print(emu.item(manager:machine().devices[":sl4:arcbd:arcbd_tms"].items["1/00000000-0000ffff"]).size)
print(emu.item(manager:machine().devices[":sl4:arcbd:arcbd_tms"].items["1/00000000-0000ffff"]).count)

emu.item(manager:machine().devices[":sl4:arcbd:arcbd_tms"].items["1/00000000-0000ffff"]):write(0,255)
emu.item(manager:machine().devices[":sl4:arcbd:arcbd_tms"].items["1/00000000-0000ffff"]):write(1,0xaa)
emu.item(manager:machine().devices[":sl4:arcbd:arcbd_tms"].items["1/00000000-0000ffff"]):write(2,0x55)

print(emu.item(manager:machine().devices[":sl4:arcbd:arcbd_tms"].items["1/00000000-0000ffff"]):read(0))
print(emu.item(manager:machine().devices[":sl4:arcbd:arcbd_tms"].items["1/00000000-0000ffff"]):read(1))
print(emu.item(manager:machine().devices[":sl4:arcbd:arcbd_tms"].items["1/00000000-0000ffff"]):read(2))