Thanks Roman. But this is a Searchable field so indexing its data as part of the indexed document's _content field is not an option. Ultimately I need to run Lucene queries such as
myfield:555. This is why I need the values to be space-spearated because Lucene treats space-separated integers as a special case. Unfortunately it appears Kentico's default treatment for multiple value fields isn't compatible with this Lucene feature.
I think my best option will be to handle the document save event an re-format this field into another hidden field then index that one. Shame, because if I could just use a space separator this would be much easier!