That actually sounds really hard to add to Blast
Actually it's really easy, so long as you forget the task of adding calls to #REGISTER_OBJECT to every raw object

Warning, warning... very poor code follows
# {ADD_TAGS;ITEM:EXAMPLE;[EXAMPLE][TAGS]}
my %ADD_TAGS = {};
$TEMPLATES{'ADD_TAGS'} = sub {
my ($object, $tags) = @_;
my $oldtags = $ADD_TAGS{$object};
$ADD_TAGS{$object} = $oldtags . $tags;
return "";
};
# {#REGISTER_OBJECT;ITEM:EXAMPLE}
$TEMPLATES{'#REGISTER_OBJECT'} = sub {
my ($object) = @_;
my $tags = $ADD_TAGS{$object};
return $tags;
};Your soulution sounds workable, but one addition should be made, the ability for an addon to override a vanilla file.
The best way to do this (that I can think of) would be a special kind of addon, eg you would have normal addons and overide addons, overide addons would be merged with the base before parsing and otherwise ignored. This would allow you to make advanced edits to a base without needing to distribute everything. For example my saurian mod removes the playable race tag from the dwaves and doubles the value of all metals, as of now there is no way for blast to change that short of a new base. With override addons I could just override the two files in question and leave the rest up to the default base.
As for material templates, there are some other tags that may be useful, namely UNDIGGABLE as well as things like tempature info.
I didn't think about the incompatability problem with tech classes, Maybe the ability to remove specific reactions/buildings from a tech class? The alloy furnace is the simplest case, some of the reactions BD adds are in direct conflict with vanilla ones.
The ability to override/remove a specific reaction would also be invaluable for addons to other blast mods, say you want to add a bugfix to said steampunk mod you could remove just the bugged reaction and register the fixed replacement in its place. With such abilitys you could just post a small patch, much easier than having to distribute a whole new copy of the mod (if the mod is large, like say Masterwork).