Good question James.
I think that whilst the stylistic influence of Hakko Ryu and Daito Ryu may be more obvious than the Shaolin influence - the existence of Shorinji Kempo owes more to the founder's Chinese experience. Were in not for that experience, Kongo Zen/Shorinji Kempo would not exist today at all. As a result, it is that element of the history that is given 'credit' for its foundation and therefor figures more prominently in the teachings.
The noble lineage to which you refer is by no means a secret, or something we'd rather not talk about. If you've come upon reticence over that subject in the past it probably has more to do with a lack of knowledge on behalf of the individual in question rather than any conspiracy to conceal or avoid the facts.
Hope that helps.
Later,
George Hyde
UCL, ULU, SOAS Dojo
British Shorinji Kempo Federation