Code a weapon attachment and include the code:
scope=1; // scope=0 causes an error message
disabled = 1;
The attachment(s) will be visible in the Arsenal.
The above code does work, BUT only when used in conjunction with ASDG Joint Rails, due to Robalo's changes that he made to how the Arsenal works (hiding scope=1/scope=0 items). Maybe BI should incorporate his changes to allow this same behaviour by default?