You could use a Chrome Managed Policy to accomplish this
{
"DefaultSearchProviderEnabled": true,
"DefaultSearchProviderName": "Custom Search",
"DefaultSearchProviderKeyword": "search"
}
{
"SearchSuggestEnabled": false,
"DefaultSearchProviderEncodings": ["UTF-8"],
"DefaultSearchProviderAlternateURLs": [],
"DefaultSearchProviderSuggestURL": "",
"DefaultSearchProviderImageURL": "",
"DefaultSearchProviderNewTabURL": "",
"DefaultSearchProviderImageURLPostParams": "",
"DefaultSearchProviderSuggestURLPostParams": "",
"DefaultSearchProviderSearchURLPostParams": "",
}
On Windows, the following registry keys should work
[HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Google\Chrome]
"DefaultSearchProviderEnabled"=dword:00000001
"DefaultSearchProviderName"="Custom Search"
"DefaultSearchProviderKeyword"="search"
"SearchSuggestEnabled"=dword:00000000
"DefaultSearchProviderEncodings"="UTF-8"
"DefaultSearchProviderAlternateURLs"=""
"DefaultSearchProviderSuggestURL"=""
"DefaultSearchProviderImageURL"=""
"DefaultSearchProviderNewTabURL"=""
"DefaultSearchProviderImageURLPostParams"=""
"DefaultSearchProviderSuggestURLPostParams"=""
"DefaultSearchProviderSearchURLPostParams"=""