... or if this was something I made up myself or pulled from somewhere else.
It's very most likely something you made up yourself, perhaps after first reading that
GURPS TNS bulletin. There are a few reasons why I strongly suspect this, each linked to the other.
First, you cannot make an offer to "
the" Aslan. There's no central government in the Hierate, the Tlaukhu is more of a debating society than government, there are no superpower clan or clans who can do whatever they like while ignoring or quashing all the others.
Second, because the Imperium couldn't make an offer to "the" Aslan, it would have to make an offer to several Aslan clans. Whether each clan would accept the offer is debatable because of the nature of Hierate politics. As are Absent Friend Hans repeatedly reminded us; the Hierate is a 5,000 man game
Diplomacy. A clan marshaling forces making a major move against the Solomani would necessarily leave itself open to elsewhere in Hierate to other Aslan.
When the Yerlyyaruiwo ambassador was gunned down by Dulinor in the Throne Room, that clan, despite being the most powerful in the Hierate, didn't dispatch battle fleets in response. Instead they sent lone assassins to avenge their ambassador. You can claim they did so due to Aslan cultural mores but the other, perhaps more realistic, reason was that such a action would weaken the Yerlyyaruiwo too much relative to their myriad enemies elsewhere in the Hierate.
Third, because the Imperium would have to make the offer to multiple Aslan clans and because the Imperium would want to ensure such offers would reach their recipients, the Imperium would have dispatched
multiple offers to
multiple recipients via
multiple methods. There wouldn't be one courier carrying one message.
The
OTU's "historical fact" that multiple Aslan clans didn't attack Solomani worlds during the Rim War wouldn't have been the result of single courier being lost.
Finally,
GDW knew their own setting and, as wargame designers, were autodidact historians. They would have know that more than one Aslan clan would have to be approached and the would have know that more than one courier would have to be dispatched. They wouldn't have sent one courier with one message to one clan.
The notes you found are very likely an adventure idea you developed after reading that
TNS blurb.