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I've found an old post from about a decade ago stating that 'The current DCAL does not allow associative hatch'. .... But that won't stop me tying! I suspect that the hatch might be set up with a series of entity attributes, or maybe it's some undocumented fields in the entity definition. This is not a must have for me, but it would be nice. Does any body have any advice regarding this (i.e. am I wasting my time even looking, or can somebody point me in the right direction) ?

After a bit of time examining objects and their attributes I came to the conclusion that it is not easily possible to create an associative hatch by using DCAL to add attributes to a polyline entity: It may be possible to use a variant record to set up the string for the HATCHDEF attribute (which is atr_str, but appears to used to store scale, angle, origin etc in a binary format), but the HATCHSCANLIN attribute is of type 8 which is not documented in the DCAL manual. Even if you did manage to reverse engineer the required settings, there would always the the possibility of a change in a future version of DataCAD which made the macro incompatible (I'd be particularly nervous about the string used to store binary data in this respect).

It is possible to create an associative hatch by using ent_copy to copy an existing hatch and then updating/adding/deleting polyverts of the new entity to define the new area to be hatched.
Using this method a macro could create associative hatches, but only by getting the user to identify an existing hatch with the required attributes ... this technique may be useful in some circumstances.

My biggest issue was that hatches created with hatch_mode had snapping points for every hatch line: I did find a solution to that, but it is a slightly different topic to this so I will post my findings in that regard in a new thread.

Thanks Dave,
I can see how that works. ...

... except your record definition does not seem to work in DCAL.

just clutching at straws here, but perhaps DCAL aligns longints with double word boundaries or something like that? At any rate when I use your definition it doesn't seem to align with the actual.

I'm sticking with what I had in my previous post for now: it works, and I can understand how it relates to your definition (although I don't understand why I am seeing 272 as the version).