Is it legal to play Matsu Gohei instead Matsu Tayuko (which is Soul of Matsu Aoiko, which is Soul of Matsu Gohei)?
Soul of...
A topic from AEG's L5R CCG Rules forum, copied for Oracle of the Void from a copy of the forum saved in October 2015, so pages it didn't save are missing. Players' names are left out; the rules officials are named. Rulings are as they stood on these dates; later rulings, the rulebooks and the Rules Archive may differ.
Is it legal to play Matsu Gohei instead Matsu Tayuko (which is Soul of Matsu Aoiko, which is Soul of Matsu Gohei)?
Sorry, sadly, no.
The "Soul of" rule is that you may only proxy the named card for the one upon which it's named, not any along the "chain". This was established with Bayushi Irezu in EE, wherein it was ruled you could only use Kwanchai as his proxy, not Tangen.