On page 196 of the PHB, it says
A target with total cover can't be targeted directly by an attack or a spell, although some spells can reach such a target by including it in an area of effect. A target has total cover if it is completely concealed by an obstacle.
As you stated, Manifest Echo is not an attack or spell, and so by RAW it is not blocked by total cover.
A Target By Any Other Name
If a DM wishes to rule as if Manifest Echo were a spell, it is unclear. Manifest Echo does not use the word 'target' - what is the target of the spell? This question, What counts as a target for a spell?, goes over some of the details, and a key takeaway is that 'target' is not a rigorously defined keyword, and every spell or ability must be judged by the DM's best discretion.
This sage advice deals with casting spells across a Wall of Force. When asked about Conjure Fey Crawford replies that
Unless a spell says otherwise, you can't cast it at someone or something behind total cover.
When asked about using, Misty Step to pass through a Wall of Force, Crawford says this spell 'targets' the caster, and that the spell requires line of sight, but not a clear path to the destination. The only conclusion I can draw from this is the same as above, that exactly what a spell targets is murky. If you rule that Manifest Echo does not target the empty space, then it can be manifested without issue. However, even if it does, there's one more possible point of contention:
Now You See Me
Page 204 of the PHB has rules about targeting.
A Clear Path to the Target
To target something you must have a clear path to it, so that it can't be behind total cover.
If you place an area of effect at a point that you can't see and an obstruction, such as a wall, is between you and that point, the point of origin comes into being on the near side of that obstruction.
Page 7 of the PHB calls out the permeability of walls again:
Specific Beats General
If a specific rule contradicts a general rule, the specific rule wins.
For instance, an adventurer can't normally pass through walls, but some spells make that possible.
For Wall of Force, two lines in the spell jump out to me as especially relevant:
An invisible wall of force
Nothing can physically pass through the wall.
The fact that the wall is invisible is more specific than the normal rules for total cover, and so can overrule them, and the targeting rules specificity about visible objects suggest that invisible objects may function differently. The spell explicitly calling out that nothing can physically pass through the wall suggests that it is possible for effects to pass through the wall non-physically, as described by this answer Do Gaze Effects work through a Wall of Force?.
Putting all of this together, I would, and have, ruled at my tables that effects that only call for line of sight, and do not explicitly involve anything passing from one side of the barrier to the other, may traverse invisible barriers such as wall of force. Fireball would not be allowed, since it creates "A bright streak flashes from your pointing finger to a point you choose within range", which would involve physically crossing the Wall of Force. But for Manifest Echo, which only requires sight, I would allow it.