To answer the original question: Yes, you can flash without a SIM Inserted, it is actually safer to do it that way, but only just.
I also have the T999L and had the Galaxy Exhibit (formerly Exhibit II 4G), the thing to keep in mind is that though they are close they are not exact and keep to the ROMs/Modems/Firmware for your variant. Though with that being said, most of the T999 ROMs (NOT modems or firmware) will work without issue on the T999L and even the I747 (AT&T Variant). Most of the major differences, and bricking troubles, come from flashing the wrong Firmware or modems, either the wrong version or wrong device/variant. I am using d2tmo version of CM10.2.0 on my T999L, any of the d2tmo CM ROMs will work just fine.