No, providing a placeholder answer is against our rules. We have absolutely no problem with you taking your time (we all very much appreciate that it does take time), but placeholders are problematic on a few levels:

1. Placeholders don't necessarily get followed up, for any number of entirely understandable reasons - this means that the comment might just be sitting there forever. As a rule of thumb, are rules are generally designed to prevent in-thread clutter before an answer is provided, not least because short, early comments are more likely to accrue upvotes than a longer, better but later answer.
2. We can't evaluate them, as they lack the substance to evaluate (and we don't necessarily know enough about a user's expertise to be able to judge whether they can deliver on the promise). This can be particularly problematic if the placeholder sparks discussion or further questions before the whole answer is ready.
3. Seeing a placeholder may discourage other users from attempting their own answer, both because they think someone else is already on it (even if, per point 1, they might not be) but also because they judge their answer will be less visible.
4. It would lead to a higher moderation burden, as it would add a significant number of borderline cases for review (as it stands, we can delete most low-effort, 1-2 sentence posts without too much concern). This is particularly the case if we would then need to periodically review placeholders to see if they were indeed followed up eventually.

I'd just stress in addition to my colleagues notes, and especially in conjunction with your edit, there is nothing preventing someone from posting an answer a few days late, only posting a note stating they will do so (for those reasons already mentioned). Threads only archive after 6 months. And if you are simply concerned about visibility of posting an answer a week after the thread had faded away, politely PMing the OP to ask them to repost the question is often a path we recommend.