custom filter is not worked on github pages

[vreality64]

github pages is building jekyll in safe mode. It means custom plugins in _plugins directory are ignored because of overriding config value of _plugins dirctory.

here are code of github pages

# Options which GitHub Pages sets, regardless of the user-specified value
#
# The following values are also overridden by GitHub Pages, but are not
# overridden locally, for practical purposes:
# * source
# * destination
# * jailed
# * verbose
# * incremental
# * GH_ENV
OVERRIDES = {
  "lsi"         => false,
  "safe"        => true,
  "plugins"     => SecureRandom.hex,
  "plugins_dir" => SecureRandom.hex,
  "whitelist"   => PLUGIN_WHITELIST,
  "highlighter" => "rouge",
  "kramdown"    => {
    "template"           => "",
    "math_engine"        => "mathjax",
    "syntax_highlighter" => "rouge"
  },
  "gist"        => {
    "noscript"  => false
  }
}.freeze

how can i apply custom filter? jekyll's description is little bor ambigous.

[vreality64]

I think this code may be hint. config["gems"] are initialized with DEFAULT_PLUGIN values

# Given a user's config, determines the effective configuration by building a user
# configuration sandwhich with our overrides overriding the user's specified
# values which themselves override our defaults.
#
# Returns the effective Configuration
#
# Note: this is a highly modified version of Jekyll#configuration
def effective_config(user_config)
  # Merge user config into defaults
  config = Jekyll::Utils.deep_merge_hashes(MERGED_DEFAULTS, user_config)
    .fix_common_issues
    .add_default_collections

  # Merge overwrites into user config
  config = Jekyll::Utils.deep_merge_hashes config, OVERRIDES

  # Ensure we have those gems we want.
  config["gems"] = Array(config["gems"]) | DEFAULT_PLUGINS
  config["whitelist"] = config["whitelist"] | config["gems"] if disable_whitelist?

  config
end