(Sorry, I was rummaging around in the basement again.)
The way I see it, drafted Merchant service is assignment to subsidized line.
Government subsidy of ship building could also imply something like a fleet auxiliary which is partially staffed by individuals subject to a draft.
This is supported in canon: "Subsidized merchants are also subject to mobilization (and use as auxiliaries) in the event of emergency or hostilities." -
TTB, "Subsidies," p 52
I too use drafted Merchants as members of a subsidized liner crew in a planetary government-sponsored merchant fleet; in
Book 7 terms I consider a subbie fleet to be a "fledgling line" for purposes of career resolution. As such, they are indeed subject to call-up "in the event of emergency or hostilities."
If you consider any of the Paranoia Press books to be canon - I can't recall officially and I'm too lazy to look it up right now - then
Merchants & Merchandise (pp 10-11) adds an interesting wrinkle: naval reserve commissions for merchant academy graduates.
So, per
Book 7 (p 17), only members of megacorp and sector-wide merchant lines can attend their company's academy, and I give these officers the equivalent INR rank as a perq. For subbie crews in "fledgling lines," who cannot attend a merchant academy by
Book 7, merchant officers nevertheless receive the equivalent rank in their
planetary navy instead.
Reserve naval officers, IMTU, may once per term roll for a skill on the Staff Officer table in
Book 5, with modifiers as per their reserve rank, to represent ongoing training.
I have an anomaly in my campaign: a "quadrant line," which services subsectors A, B, E, and F. Tri-Star Line is treated as a subsector-line per
Book 7 for positions, assignments, bonuses, and mustering out, but its merchants ARE allowed to attend a corporate academy and receive reserve commissions in the provincial (subsector) naval fleet on graduation and qualification.
So, taken together, per
Book 7 and
M&M . . .
Which gets me back to the original question: is their an Imperial equivalent of the merchant marines?
. . . I don't interpret the rules that way, no.; rather, the Imperium commissions some merchants as reserve naval officers instead.