"relatively easy" is relative. Where did you get the FWL info? (just curious, I don't get to delve into enough fluff ... dang kids and other rl stuff)
The Perseus entry in FMFWL and TRO3067. The Perseus is based on the Orion and there's no mention of the it being a difficult conversion, unlike the Night Chanter entry in TPFoster.
And this reminds me, many of the Kuritan OmniMechs were based on Inner Sphere chassis: The Owens from the Jenner, the Strider from the Cicada, the Firestarter Omni from the Firestarter, the Blackjack Omni from the Blackjack... These all were done in relatively quickly, especially considering that the Kuritans aren't exactly technical wizard and were working with captured tech, not something they developed on their own (though i suppose the Clans had troubleshooted most Omni stuff centuries ago).
So, i suppose the actual base model matters a lot. So it becomes a question what options would the Blood Spirits have available, perhaps.
Of course, as you point out, being constantly under attack makes developing new stuff tricky.
As it happens, the Spirit Walker is based on the Black Knight... And good news for the Blood Spirits, they get it, until their very end.
You can see some of the reasoning as to why the Orion was never really updated to an OmniMech in the Orion IIC entry in TRO: 3060.
The Perseus is an OmniOrion, but i suppose the Orion was too revered among the Clans, not like the FWL has such inhibitions.